As a native Londoner and a student of 19th-century British popular fiction, I should like to add a footnote to the background article on the New York City Opera production of ''Sweeney Todd'' (Oct. 13).
The novel, ''The String of Pearls,'' was written by the prolific Thomas Peckett (or Preskett) Prest, author of an even more horrifying novel, ''Varney the Vampire, or, the Feast of Blood'' (1847). Prest seems to have drawn on a London legend, which was also used by Charles Dickens, who in ''Pickwick Papers'' (1836) had Sam Weller tell Mr. Pickwick a toned- down version of the story in which the pies were made of kittens', not human flesh. Dickens had also heard a version of the cannibal pie story in early childhood and wrote about it in ''Nurse's Stories,'' one of the essays in ''The Uncommercial Traveler'' (1860). Captain Murderer, in that grisly bedtime story, was accustomed to woo, marry, murder and eat his brides, after having made them first prepare the pastry crusts into which they were to be placed. But he was finally defeated by his last bride, who saw what the captain did to her twin sister, yet bravely married the captain, made the crust and was killed and put into the pie. But she had swallowed a deadly poison before she was slain. When her evil husband ate his cannibal pie, he, too, was poisoned and ''came out in spots, and turned blue, and swelled so that he filled the room from floor to ceiling and from wall to wall, and at one a.m. precisely, child, he BUST!'' Such, perhaps, is a folkloric background to the current version of the tale of the Demon Barber of Fleet Street.