According to the archives, it was the first fictional film Birt Acres had ever made. It predates what had been the previous earliest British film in existence - Acres's footage of the 1895 Derby - by about two months.
Originally owned by Mr Williams's grandfather, George, who travelled with the circus showing it as part of his magic lantern and peep show, the film ended up in the loft of the family's engineering business in Worcestershire.
When he died in 1946, it was passed down from father to son and then onto his grandson. Mr Williams said: 'I'd always wanted to find out something about the films but didn't know who to ask.
'I tried taking them to a film museum in Brighton but it was closed so I came home, stuffed them in the back of my wardrobe and forgot about them.'
It was only when his niece, a keen genealogist, heard about them and contacted Sheffield University's Fairground Archive, that the cache of 14 films came to light.
Dr Vanessa Toulmin, the archive's director, said: 'I spend my life looking at old films so I had no reason to suspect that this was anything amazing.
'But it is astounding. It is only about 100ft of film, and it was tagged onto the end of another film. But we have had it verified by four experts. It is not only the earliest British film ever found, but the first crime film made anywhere in the world.
'Another three months and it would not have survived because the nitrate was already rotting inside the tins.'
Deac Rossell, a film historian, said: 'What is extraordinary is that there are newspapers of the day that have been used as a backdrop on the set.
'They clearly show that it was the third week in April. To give historians that kind of time check is a dream come true.'
After months of painstaking work, the film has been restored frame by frame. It is due to get its - second - world premiere at a festival for silent films in Venice in October.
