Today is the anniversary of Johnson's departure for London, March 2nd 1737. So it seemed appropriate to think about London rather than Lichfield. And as I'm writing a biography of another writer who left home and went to London at about the same time, I thought it would be interesting to reflect on what London meant for her – with half an eye on Johnson's experiences at much the same time (although 'home' isn't quite what Dublin felt like by the time Mrs P left in the winter of 1738).

With the hindsight of history we know that when we read about J's London experiences, we're reading about the formation of a profoundly significant writer, someone whose life and works have helped define what we mean by literature, authorship, the literary life.

Nothing of that sort accompanies our reading of Laetitia Pilkington. In literary history she occupies a small, very small, rather murky place as one of the early 'scandalous memoirists'.

And yet I hope to persuade you that just as Johnson struggled through from failure as a teacher and playwright, from impecuniousness as a jobbing writer, to literary success, so too Mrs Pilkington deserves recognition for an extraordinary achievement.

Indeed, quite an astonishing achievement when you stop to think about it.

For Mrs Pilkington wasn't only poor and Irish when she arrived in London, she was also a disgraced woman. Her clergyman husband, the Rev Matthew Pilkington, having surprised her one October night with a gentleman in their bedroom – not himself – and having made sure that 12 night watchmen accompanied him as witnesses, threw her out of the house, and instigated proceedings in the Consistory Court for a formal deed of separation.

It's one of the commonplaces in writing about women in the 18th century that there was no way back from the loss of sexual reputation. But in fact Mrs Pilkington turned her disgrace into an enduring work of literature, one that has been described as 'a minor classic of the 18th century'. In writing her Memoirs, she devised a wholly original genre. And she was one of the initiators of a form that is now entirely familiar, whose other early exponent was: Sam Johnson

I mean literary biography, specifically, the writing of the life of a literary figure by a friend or intimate acquaintance, someone who, like Johnson when he wrote the Life of Savage (1744) had walked the streets with the man and been in constant communication. Mrs Pilkington was the first to publish anecdotes of Jonathan Swift, based on her own and her husband's years as protégés of the great Dean of St Patrick's Cathedral, Dublin.

Swift was probably, along with Pope, the most famous literary figure in the kingdom –certainly, without question, he was so in Ireland. But in England too. He had been a thorn in the government's side, publishing anonymous pamphlets that everybody knew were by him, such as the Drapier's Letters (1724) which prevented the government flooding Ireland with cheap coinage. Swift was a hero to the Irish. Eccentric, angry, brilliant, he was a mythic figure: stories about Swift were constantly circulating, while his presence in the cultural imagination was firmly established by the runaway success of Gulliver's Travels (1726).

Johnson's early London writings owed a good deal to Swift. When Johnson began writing the Parliamentary Debates for Edward Cave's Gentleman's Magazine – supposedly reports of speeches actually given in parliament from Nov 1740-Feb 1743 – he took Swift's Lilliput, used Swift's fiction, for fact-fictions of his own. Walpole's government had banned reports on House of Commons debates so the Parliamentary Debates were published as 'Debates in the Senate of Magna Lilliputia'.

Two 1739 pamphlets are satires in the vein of Swift: elaborate indirect irony. Marmor Norfolciense – the Norfolk Marble – and The Complete Vindication of the Licensers of the Stage. The Complete Vindication adopts the voice of one who agrees with control of the stage to actually argue for freedom of speech. This is like the Swift of A Modest Proposal which modestly proposes that the best way to deal with famine and beggary in Ireland was to start eating the children – something which, symbolically, the British government's rapacious policies towards Ireland was already doing.

It's helpful to remember what an important figure Swift was to the generation of writers that came after. When Laetitia Pilkington arrived in London, sometime towards the end of 1738, about 18 months after Johnson, she brought with her a considerable asset –pretty much her only capital – her inside knowledge of a man whose name every literate, or 'polite', person knew.

Let me try to sketch for you this Irish woman of whom I've come to think ever more highly the more I've worked on her. She was still relatively young in 1738, not quite 30 (same age as Johnson) though she had been married for 13 years and had given birth to 6 children, 3 of whom survived. They belonged, of course, to her husband and he ruled that she was not to see them – married women had no legal existence and could not own property including children. (The rights of ex-married women disappear into infinity.)

Physically, she was very small, possibly not much above 4 feet tall. In London they called her 'the little Irish muse'. She was a voracious reader and snappy talker, a wit in an era that prized wit, and a poet. Precocious as a child, she was the eldest daughter of a fashionable Dublin doctor-midwife. Her social circle included some of the best families of the Anglo-Irish Protestant Ascendancy. The wealthy Bishop Clayton, for example, with his opulent house on St Stephen's Green, was godfather to her youngest son Jack. When she married Matthew Pilkington, a curate under Swift, she had married a little beneath her. On the other hand, he, like her, was an individual of considerable talent, energy and drive, determined to make something out of the opportunities available. He was also small like her. Charming, fast talking, passionate about literature and the arts in general, a musician and a poet, Matthew made an impression and Swift took him very seriously as a protégé worth helping and encouraging.

Around Swift in Dublin in the 1720s and early 1730s a lively literary culture flourished. There was Thomas Sheridan, grandfather of Richard Brinsley; Dr Patrick Delany, later to marry Mary Delany whose diaries and letters are an important source of info for the period; Mary Barber, whose poems Swift helped see into print; Constantia Grierson, poet and classicist, who died young; and numerous others. In this circle, which met frequently, Matthew was 'mighty Thomas Thumb' and Laetitia was 'her Serene Highness of Lilliput'.

Her Serene Highness of Lilliput, endlessly pregnant, was jealous of her husband, especially after, with Swift's help, he published a volume of poems, and then, with Swift's further help, or 'interest', was given the one-year position of chaplain to the Lord Mayor of London. (John Barber, Swift's printer.) Off Matthew went to London (Sept 1732) all agog to see the sights and be received by Pope at Twickenham. And did he want his wife to go with him? No he did not. She would cramp his style, he told her. Matthew had every intention of living the high life: on the boat over he attached himself to Edward Walpole, the PMs somewhat dissolute son, and the moment he hit town he was at the theatre. Soon he was spending all his spare time paying court to the actress, Mrs Heron.

Matthew's year in London (18 months) ended badly. Swift used him as an agent for his own inflammatory poems – printed anonymously but, again, it was well known they were Swift's – and he was arrested, along with Mary Barber, the printer and the publisher. Miserable, he blabbed, and the word got back that he had betrayed the Dean. Matthew's name was mud in Dublin, his hopes of preferment in the church at an end; meanwhile, he was also disenchanted with family life.

Was she having an affair? We don't know. I hope so. Does it matter? It used to matter terribly. As Mrs Pilkington discovered, the sexual double standard was vicious and remorseless. We have only to consider the subsequent fates of the two people in that bedroom: Robin Adair, whose sex appeal was legendary (so much so that it gave rise to a song, Robin Adair) a man who 'played the devil with the ladies', went on to marry into the aristocracy and rise to be Surgeon General of His Majesty's forces.

Mrs Pilkington, banished from her home at 2 in the morning, was pursued by all the brothel keepers, all the bawds and procuresses, all the rakes and libertines in Dublin – for, once beyond the protection of her husband she became prey. A fallen woman was apparently anybody's. It was a profoundly shocking experience, even for someone as tough and worldly as she.

When she wrote about it later she wrote it up as farce. There are scenes in the Memoirs depicting the months following her eviction and subsequent court case that read like slapstick comedy: Mrs Pilkington chased through her lodgings by drunken youths, barricading herself in upper rooms, being beseeched through the keyhole. The reality was not amusing: these (generally well-born) young men wanted to rape her and would have done so if they could.

She got her own back in the Memoirs which were designed for revenge. Revenge upon whom exactly? These youths; and other named individuals who had insulted her, spread malicious gossip, refused to help out with a guinea. Or individuals whose behaviour was no better than hers but who got away with it, clergymen in particular. Her husband, of course, whose morals, as Pope's friend Bolingbroke observed, were somewhat lacking. Bishop Clayton, who was something of a womaniser.

But revenge in a broader sense too. Mrs Pilkington's experiences brought her into contact with many other cast-off women: prostitutes, pregnant mistresses, unwanted wives. She saw how male sexual behaviour threatened female health and happiness. This was hardly an original observation: this is the era, after all, of Hogarth's The Harlot's Progress (1732) and The Rake's Progress (1735). But she also saw how the economics of sexual subordination pressed women into dependency on men, whether as wives, mistresses or whores. Having ceased to be a wife, rejected by her brother, with no living father, having alienated her friendship circle, what exactly was she meant to do except become a mistress or a whore? How was she – and women like her – supposed to live?

Mrs Pilkington went to London to seek her fortune as a writer. She arrived with three guineas in her purse. Johnson liked to tell the story of his arrival, with 'two pence half penny' in his pocket – and David Garrick with even less.

I described her earlier as a 'disgraced' woman. This is true: disgraced, but not shamed –she was the celebrated Mrs Pilkington. And as we know from our own times, celebrity has its own laws. There was a curiosity about 'the little Irish muse' as she was now called, which it was in her interest to cultivate. She took lodgings in St James's, where the wealthiest people were and also the most important publisher, Robert Dodsley, who kept open house for writers at his shop in Pall Mall.

Perhaps, as a woman, she didn't hang out at Dodsley's. Certainly, as a woman, she could not be a member of White's Club, but White's – the most exclusive gambling, drinking club in London – was for a while the centre of her operations. Strategically, she took lodgings right across the street. She sat in her window, at her writing table, pen poised. The men of White's, lolling about, Lord Chesterfield among them, the Dukes of Marlborough, Newcastle, Devonshire etc, etc, and Colley Cibber the actor, her most important supporter, could all see her and, urged on by Cibber, engaged in frivolous games of wit with her.

I have to say something now about the kind of poet Mrs Pilkington was. She was an occasional poet, that is, she wrote poems to and for occasions. She was quick with a rhyme and an arresting point of view. Like a portrait painter, she caught a likeness that others could see but not express so well. This kind of poetry, social poetry, was held in high esteem and someone with her fluency and sharpness was an asset in circles like those of the leisured men of White's – for if you happened to be a man who was not all that quick at repartee and incisive rhyming couplets, you could get Mrs Pilkington to write some for you. Nobody need know. That was one way she got guineas. Another was by sending over verses known to be by her, which amused the company – verses making fun of someone, or praising someone. Another was by what I think of as stand up comedy: the men of White's would cross the street and visit. Mrs Pilkington would rattle off her anecdotes, her 'entertaining speech' and they showed their appreciation when they left.

She may have been a courtesan, she may have provided sexual services. She says she didn't. We have no way of knowing one way or the other but there are some telling details. One is that nobody ever suggested she had gone into keeping with any particular man; and she made a point – certainly at first – of targeting very elderly men: Colley Cibber was in his sixties. Another is that she was able to obtain support from the unimpeachably respectable, men who enjoyed her conversation every bit as much as did the habitués of White's. The Archbishop of York, for example, Thomas Herring, later Archbishop of Canterbury. Herring was a handsome man with literary interests. She went to call on him at his palace at Kensington: