Hayman's self-portrait c.1735 has been owned by the Royal Albert Memorial Museum and Art Gallery in his home city of Exeter, Devon, since 1963.
However, the other half of the portrait - a lady thought to be Hayman's estranged first wife of whom nothing was known - is a new discovery, spotted by Mr Mould's researcher, Bendor Grosvenor, in an auction in rural New Hampshire where it was described simply as 'Circle of Hogarth'.
Restoration soon revealed that the picture had been substantially over-painted, concealing not just an easel with an unfinished sketch but also the knee of a man. It was then that Mould realised he had bought only one half of a portrait.
A trip to the West Country to place the two halves of the painting side by side confirmed that they had originally been part of the same portrait - but who was the sitter? Further research revealed that Hayman had split up from his first wife shortly after he began work on the picture and its unfinished state suggests it was the artist himself who cut the picture in half. He later remarried.
It is hoped that the two halves of the picture will be permanently reunited. The gallery has applied for grants to buy the newly discovered section for an undisclosed sum.
The work will go on display as part of Philip Mould's new inaugural public exhibition Four Hundred Years of British Portraiture at the gallery at 29 Dover Street, Mayfair.
• On November 24, 2004, a Hayman work set the then record for any work sold at auction in the UK outside London when his portrait of the Richardson family sold to Agnews for a hammer price of £540,000 at John Nicholson's of Fernhurst.
By Roland Arkell
