London's Crossbones Graveyard is the setting for a very eerie Halloween celebration

Redcross way was quiet, empty, and suddenly a bit eerie. I felt quite alone in this still stretch of lane that runs between two busy roads just south of the Thames.

A plaque told me that this was the site of the Crossbones Graveyard, from late medieval times an unconsecrated burial site for prostitutes – and later, paupers – until its closure in the 19th century on public-health grounds. The "shrine" around it honoured up to 15,000 bodies buried on the site, and was, I read, a "place of healing where the Wild Feminine is honoured and celebrated for all that she is – whore and virgin, mother and lover, maiden and crone, creator and destroyer".

Whose voice was this? Who had threaded the ribbons into the wire, tied the doll to the fence, typed up the prayers, cut the lace? Who were the Londoners who felt compelled to mark a humdrum space with this iconography of femininity and death? The answer, I discovered later, was the Friends of the Crossbones Graveyard, in particular its charismatic motivating force, the local writer (and creator of The Southwark Mysteries) John Constable – or rather, his alter ego, John Crow. The Friends campaign for the preservation of the site, with some precarious success. They hold a candlelit vigil here every month for the "outcast dead", and have, for the past 10 years, held a Crossbones Halloween Event – to which I went last year, curious and unaccountably nervous.

Constable, white-haired, pacing about in a dark robe, welcomed the hundred or so of us into a circular room on Southwark Street. His resonant voice invited us to place mementos or photographs of lost ones on a makeshift altar at the front of the room. We were each given a white ribbon with the name of one of the Crossbones dead, taken from the London Metropolitan Archives. We were to adopt their spirit for the course of the evening. At this, I felt a spasm of rationalism; a twist of anxiety at memories of school drama; a desire to bolt. Would we have to get naked? Commune with something I didn't know I wanted to commune with? I sat in a chair towards the back.

Constable swooped about in his cape. An expectant buzz mounted.

A priestess started things off, leading us in a meditative moment of humming. A single note was held, surprisingly tunefully, by the crowd. I felt a space open up in the room. My ears, my whiskers perked up; I was suddenly alert and curious. A witch broke into a rap. I squirmed, feeling a tickle of laughter taunt my belly, my nose. Someone then asked us to close our eyes and think of the dead. To think of past pain, past loss, past regret – and let these go. We each read out a word: light, compassion, generosity – things to wish for and cherish.

Constable then took over, adopting the persona of John Crow, his "trickster-shaman". Actress Michelle Watson became the Goose, a prostitute on the Bankside, a wise and sassy creature with a throaty voice, radiating erotic scorn. Together, Crow and the Goose performed, in verse and song, sections of Constable's poetry, bringing to life the women refused burial on consecrated ground by the very Church that licensed their practice.

In 1107, a stretch of land on the southern bank of the Thames was granted to the Bishop of Winchester. The Bishop's estates came to be known as the Liberty of the Clink – "Clink" because the Manor House straddled this underground prison, and "Liberty" because the Manor lay outside the City's jurisdiction. Much that was forbidden elsewhere – including theatre and bear-baiting – was allowed here, and it was in this spot that the "Geese" (perhaps so-called for the white aprons they wore, or for their white breasts bared to river visitors) could practise. Brothels, or "stewes", were licensed and regulated by Henry II's 1161 ordinance, with the Bishops collecting fees and fines paid by brothel licensees.

Crow's Goose had, in fact, visited John Constable one night in a vision. "She simply walked in," he told me, and took him for a walk in the Southwark streets. He returned home and feverishly wrote – transcribed, he says – the verses that have remained largely untouched since. In "John Crow's Riddle", Constable wrote:

For tonight in Hell

They are tolling the bell

For the Whore that lay at the Tabard,

And well we know

How the carrion crow

Doth feast in our Crossbones Graveyard.

Constable had no idea what the Crossbones Graveyard was; it was just one of many phrases that came pouring out of him in that frenzied night of writing. Some time later, Constable heard about the Museum of London cautioning against planning applications for the Jubilee Line extension works, due to post-medieval burials from the "Skull and Crossbones" cemetery being disturbed. (The museum removed 148 skeletons in excavations before the works began – less than 1 per cent of the burials, it claims.) Constable went to see the site and recognised it as one of the places the Goose had led him that hallucinatory night. Returning to the verses, he noticed the reference in the poem and felt the Goose's presence nearby – arms folded, a mocking smile.

A melée of voices intertwine in the poems: the Goose, bawdy, world-weary, wise; a Caribbean, earthy male voice; a Dylan-esque troubadour; an Elizabethan rhythm. "Liberty" is a recurring theme in both the writings and the Halloween event. The Goose, fond and defiant, writes:

And when our Lords in Westminster

Denounce my 'Impious Blasphemy',

My gob in the face of all God-fearing

Servants of His Majesty.

What though they throw me in The Clink,

Or King's Bench or Marshalsea,

And leave me there to rot, they think,

For brazen acts of harlotry?

I call upon my Bishop

As Defender of my Liberty.

When all-mighty City Fathers,

Those dread Guardians of Morality,

Do ban 'all gaming, drunkenness

And acts of gross effrontery'.

What though they thunder Over There?

It matters not a fig to me,

Over 'ere's the Ward Without

The Law of London City,

Where Whores are subject only to

Fair Southwark and Her Liberty

London's sexual history, with its episode of ecclesiastical brothel licensing, has fascinated various writers. EJ Burford wrote, in The Bishop's Brothels of 1976, that when Henry VIII annulled Henry II's ordinance, he demolished "at a stroke four hundred years of the most picturesque and picaresque history" of the Bankside. Henry II's ordinance had helped to make "Southwark the pleasure-garden of London for many centuries". The city's sexual past is seen here through misty eyes, as a liberatory and joyful state of nature, which in Constable's prose is also a state of grace.

Brothel licensing, however, has a regulatory rather than liberatory rationale: the management of syphilis – the "filthy disease" – and of the swelling ranks of prostitutes, widows and orphans of the Crusaders. The ordinance provided a measure of protection: no "stew-holder" was to "keepe any woman to boord but she to boord at her pleasure" – prostitutes could come and go as they pleased. And no "single woman" is to "bee kept against her will that would leave her sinne". But prostitutes were forbidden from having a "paramour". Punishment for some freely chosen pleasure with a lover included a hefty fine, three weeks in prison, a session on the "cucking-stool", and ejection from the Liberty.

The Crossbones campaign celebrates a strong, elemental and witchy female sexuality; it thumbs its nose at a feminism that has cast prostitutes as victims of misogyny and coercion. And this it has in common with the growing public voice of sex-worker campaigns, which have enthusiastically embraced Crossbones and conduct tours of the site. At the Halloween event, a representative from the International Union of Sex Workers (IUSW) spoke about its campaign to have the Crossbones site earmarked as a memorial for sex workers. The organisation likens the prostitutes' regulation by the Church and their exclusion from Christian burial to the contemporary semi-legality of prostitution in the UK.

The IUSW, like many sex-worker campaigns, insists on sex work as autonomously and freely chosen, and defiantly requests a dignity denied to sex workers both by puritanical right-wing moralism and by puritanical feminism. The right to autonomous sexual self-expression, whether commercial or not, is key not just to libertarianism but also to the post-feminism that has become the core of contemporary young femininity. Here, feminism is cast as having achieved its aims – namely, the equality of men and women. Young girls are therefore free to enjoy their sexuality without worrying about the political ramifications of its expression. This is the language Crossbones speaks in, most eloquently through the "Whores d'Oeuvres" at the Halloween event: two women in basques who schooled us in the spiritual potential of our pelvic floors. We held hands with our neighbours, breathed and moved our hips in unison – a lesson in spiritual burlesque.

These harnessings of the past to contemporary sexual politics made my feminist and historical antennae twitch. A historical Liberty which denied women numerous viable means of economic independence, which punished women for their very own pleasure, and protected them under a regulatory system entirely at the discretion of brothel-keepers and Bishops, isn't much of a Liberty.

And yet, as I breathed in unison with 100 others, under the tutelage of the Whores d'Oeuvres, I laughed, and felt humbled – holding hands with strangers, and allowing the centrality of sex to be fully, joyfully, playfully acknowledged. In Redcross Way, where we processed with candles, Constable made a gin offering to the Spirit of the Goose. I closed my eyes and thought about desire – its power and its vulnerability; the vulnerability of sex workers; the spectre of violence and murder always hovering over the figure of the prostitute, the shadow of the Ripper never far away. The shame associated with sex work; the vulnerability that desire confers on all of us.

Later, over coffee in Borough Market, Constable spoke to me of "Liberty" not as a political or historical reality, but as a state to which we might aspire, "in this life or a future life", a place where all ideas can coexist.

Back in Redcross Way, I drank from the gin being passed around. I tied my ribbon to the fence. The Goose, I thought – this Goose – is Loose. Whatever the Liberty. For details of tonight's Halloween ritual, see crossbones.org.uk