"He kept a shop in London Town /Of fancy clients and good renown/And what if none of their souls were saved?/They went to their maker impeccably shaved."
So goes "The Ballad of Sweeney Todd" in Stephen Sondheim's popular musical. For two centuries, newspaper-readers, theatre-goers and young children have been repelled and entranced by the exploits of the Demon Barber of Fleet Street; a mass murderer who slit the throats of his clientele as they relaxed in the barber's chair. Their corpses were then served up by his lover in apparently highly regarded meat pies.
The exact number of smooth-chinned gentlemen despatched by Mr Todd - and, indeed, whether or not he existed at all - is disputed among crime historians. Tonight, BBC1 airs Sweeney Todd. Set in the backstreets of late 18th-century London, it treats the murders with a grim realism. The actor Ray Winstone plays the leading part, while Essie Davis is Mrs Lovett, a girlfriend who "isn't too choosy about her men or the source of meat for her pie shop".
A high gore content and body count are assured. More questionable is Winstone's claim that Todd is "a character you may find yourself feeling sorry for".
But is it really possible that such a picaresque psychopath ever existed? Crime historian Peter Haining, who ploughed through the available evidence for 25 years before writing Sweeney Todd: The Real Story of the Demon Barber of Fleet Street in 1993, believes so. He was absolutely convinced of Todd's existence by reports he found in The Newgate Calendar - "a more factual, reliable document than the penny dreadfuls of the time". He added: "It is simply all too gruesome not to be true."
By Mr Haining's account - still not widely accepted - the man was the monstrous product of his hard upbringing, an opportunist thief who was barbarous even by the standards of his contemporaries, "polishing off" at least 160 victims in a 17-year killing frenzy.
The story begins with Todd's birth, to gin-soaked parents, in a Stepney slum on 26 October 1756. He endured a short, poverty-stricken childhood (during which his main fascination was with the instruments of torture at the Tower of London) before being orphaned aged 12. He survived by becoming apprentice to a vicious cutler (specialist in razors), John Crook. Two years later, he was jailed for petty theft and sentenced to five years in the notoriously harsh Newgate Prison. There he learnt his trade as "soap boy" to the prison barber. By the time he walked out of the gates in the autumn of 1775 he was "a morose, bitter and cruel young man of 19", according to Haining. He then found work as a "flying barber" of no fixed abode, before settling in the infamous premises next to St Dunstan's Church on Fleet Street, then a haven for drunkards and robbers. "Easy shaving for a penny - as good as you will find any," ran the shopfront sign, next to which were displayed jars with teeth he had pulled and blood he had let - references to the surgical duties of a barber.
In Haining's account, Todd's first reported victim was a "young gentleman from the country" who fell into conversation with the barber on a street corner. "The two men came to an argument and all of a sudden the barber took from his clothing a razor and slit the throat of the young man, thereafter disappearing into the alleys of Hen and Chicken Court." As evidence Haining quotes the Daily Courant of 14 April 1785, which reported the murder with horror and fascination.
It was soon after that the legendary trick barber's chair supposedly came into use. Customers unlucky enough to find themselves in a dark, empty shop and unwise enough to flaunt their wealth met their grisly end beneath the floorboards. Todd pulled a lever, which tipped the victim headfirst through a revolving trapdoor on to the cellar floor (there was an identical chair bolted to the other side of the trapdoor which swung up, to allay the suspicions of passers-by). He would then descend the steps to cut their throat from ear to ear.
Todd initially left the bodies in the underground passages below the church but, worrying about the rapidly growing pile, instead devised the idea of employing Margery (some say Sarah) Lovett's piemaking skills.
He dismembered the bodies, stripping the flesh, heart, liver and kidneys into a box to carry to Mrs Lovett's bakery at nearby Bell Yard. Bones and heads were piled in the Weston family vault beneath the church - later to be discovered by unfortunate detectives, tipped off by churchgoers upset by the stench of rotting flesh (which must have been overwhelming to be noticed above the festering stink of Georgian London).
Gossip about disappearing sailors eventually led to the pair's arrest. Lovett confessed everything before committing suicide in prison, apparently, while Todd's trial in 1801 is said to have generated feverish excitement. He stood accused of just one murder - enough to hang him, if convicted - that of a seaman, Francis Thornhill, on his way to deliver a string of 16,000 pearls when he decided to stop for a shave. A pawnbroker's clerk gave evidence that the pearls were later pawned by Todd and the prosecution described how his house "was found crammed with property and clothing sufficient for 160 people" - causing a "thrill of horror" to run round the court.
The man Haining believes was the Demon Barber blamed his mother: "I was fondled and kissed and called a pretty boy," went his dubiously reported testimony. "But later I used to wish I was strong enough to throttle her. What the devil did she bring me into this world for unless she had plenty of money to give me that I might enjoy myself in it?"
The jury took 10 minutes to find him guilty and he was hanged, aged 45, on 25 January 1802 at Newgate Prison, in front of a crowd of one thousand.
Gruesomely thrilling stuff - but complete cobblers, according to others. The playwright Christopher Bond, whose 1973 work was adapted by Sondheim for the musical, began his tale by telling readers: "Sweeney Todd is pure fiction ... No one has ever succeeded in finding a shred of evidence as to the existence of a Demon Barber thereabouts."
What Mr Haining presents as truth may be the colourful imaginings of the day's tabloids - the populist 19th-century "penny dreadfuls". The cheap sheets were the first to seize the story, zealously reporting Todd's "trial" and execution as fact, alongside vivid descriptions of the decomposing human remains. The strong likelihood is that they grossly exaggerated reality to boost sales. The stories spread through word of mouth and certainly suffered embellishment along the way.
Another favourite villain of the time was "Spring-Heeled Jack" - a mad nobleman who got his kicks from skulking around London in the middle of the night and leaping out on young women and old men to scare them.
He was said to breathe flames and to have springs in the soles of the boots so that he could jump away from his pursuers.
The authorities eventually tracked down the practical joker and warned him off, but his behaviour had by then become something of an urban myth and inspired copycats.
Later, there grew the story of Jack the Ripper - a villain whose existence and identity is contested even more fiercely than Todd's. The novelist Patricia Cornwell is convinced that he was the painter, Walter Sickert - to the outrage of other "Ripperologists".
The hack Thomas Peckett Prest wrote the most popular story about Todd and Lovett, The String of Pearls (1846), in which he praised her "delicious" pies. George Dibdin Pitt adapted it for the stage and the myth grew. The Demon Barber's first cinematic appearance was in a silent film in the 1920s and he soon got speaking lines in a serious horror film, Sweeney Todd: The Demon Barber of Fleet Street, in 1936. The Sondheim musical first ran on Broadway in 1979 - the sudden draw of the knife across the victim's throat left the audience gasping - and won nine Tony Awards. Ben Kingsley and Joanna Lumley starred in the most recent television version, in 1998.
Joshua St Johnston, who wrote the BBC1 drama to be screened this evening, said he wanted people to believe the story was true, but admitted even he was sceptical.
"Researching was very confusing," he said. "In the end it was only by visiting St Dunstan's Church, where Sweeney Todd was meant to have hidden the bits of the bodies that didn't go into pies, that I realised he probably didn't exist, as there was nothing there referring to it."
Mr St Johnston hopes his version moves the tale beyond music-hall melodrama: "It's an attempt to apply a 21st-century understanding of criminal psychology to an 18th-century serial killer."
That little evidence exists of Sweeney Todd may be down to how easy it was to get away with murder, and not because he was the product of gruesome Georgian imaginations. "Maybe there really was a Sweeney Todd after all - he just never got caught," admitted Mr St Johnston.
Two centuries on, it is unlikely that the truth and legend of Sweeny Todd will ever be adequately disentangled. But it seems certain that our appetite for the gruesome will always be satisfied by the Demon Barber's capacity to titillate and revolt.
"The tale of Sweeney Todd has the same lure as the Jack the Ripper story," said Mr Haining. "It has, if I can use these words, the perfect ingredients: the revolving chair, the men's eventual fate as pie filling, and the enduring mystery of not being sure how many he killed."
Or to steal the words that Anna Pavord penned for The Observer in 1979: "Sweeney Todd will never die. We all need bogeymen and he was bogier than most."