Sweeney is first heard of in one of the periodicals known as 'penny dreadfuls', primitive in execution to make them affordable to the masses. This particular penny dreadful was entitled The String of Pearls and was published in serial form from 1856-7. The author was Thomas Prest. It told of the barber who would kill his customers and keep their bodies underground until the flesh could be removed and brought to the adjoining pie-shop of Mrs Lovett, who used it in her comestibles. There have been various other adaptations including the recent musical film version with Johnny Depp in the title role.
Did Prest have an historical source? It was averred in Tell-Tales that Fouché of the French police had chronicled such a case in the Rue de la Harpe in Paris, but Fouché's original version has defied discovery. However, it is possible Prest used a number of sources in combination.
The Annual Register for 1784 tells of a barber who cut someone's throat in Fleet Street and this is what may have brought the London thoroughfare into the story. It was apparently widely believed in 19th Century England that pie-sellers were not always to be trusted regarding the contents of their confections. In 1818 an unfortunate pork butcher named Pizzey was almost put out of business due to an article by James Catnatch which suggested human meat was used in pies. Pizzey successfully sued Catnatch.
These episodes were probably combined to make the Sweeney Todd story, but it is just possible that the tale had more solid foundations in history that may yet be discovered.