I am a treasure hunter. I don't mean that I leave the house each morning armed with a pick and a shovel and an old map marked with an X. No. My tools are the Antique Trade Gazette's auction guide, my old motor and forty-years accumulated knowledge of art and antiques. I travel the world looking for mistakes made by auction houses and dealers wherever I can find them. I eat most days, but I'm not getting rich.

Picture restorer, dealer, Mike Cunningham was one of the greatest treasures I ever found. We clicked from the moment we met and I was sure we would grow old together. When he died in his sleep in 2000 I was more upset than when I lost my Dad. Mike was fifty-two years old, fit and full of plans for the future. He had recently decided to sell his London home and retire to Hastings, on the south coast of England, where he and partner Sue already owned a small house in the Old Town. Mike and I had bought many pictures together over the preceding twenty years, most of them turned over quickly for a profit. But when Mike died we were still half shares in a painting that, if we had some provenance, would have secured our futures and that of a small African nation.

Mike bought the picture from some Irish travellers on the Goldbourne Road (off Portobello Road, London) one, very wet, Friday morning in 1980. He paid two pounds ($4.00). He didn't even know that it was a painting. All he could see in the half-light was a muddy, cupboard door, burned on one side with traces of paint on the charcoal. The other side had old wallpaper stuck to it and a letter attached to the top right-hand corner. He did think the letter looked interesting - although he couldn't speak French - and he thought he recognised the signature. Later, back at his studio in Fulham he wiped the mud from the charcoal and discovered Picasso's Guernica - in colour http://www.yopicasso.com.

The painting measures 45.5cm x 57.5cm is signed Picasso 1937 in the body of the fallen warrior. The letter on the back was addressed to Gordon Davy of the R.A.E. Cap D'Antibes 2.1.46 and signed Picasso and a footnote - Operation Special Executive Project Design - Guernica. The top left-hand corner of this letter (with "Pour Gordon" written on it) was detached and lost, but a Photograph does exist.

In July 1981 Mike showed the picture to Roland Penrose. Penrose liked the picture. He said that he had never seen it himself, but he promised he would make some inquiries. Unfortunately Mr Penrose died, before Mike was able to enter into correspondence with him.

It took me a couple of years to persuade him, but, in a moment of weakness, Mike eventually sold me a half share. In 1987 we approached a handwriting expert at New Scotland Yard and asked her to take a look at the letter. Encouragingly she saw no reason to suppose the letter was a fake, although, due to the lack of suitable reference for comparison, she was unable to give a definitive judgment. The hunt began for samples of Picasso's writing from around the same date, written with a brush and, preferably, written while he was in a similar frame of mind.

I had the brilliant idea that we should write to the Picasso committee in Paris and ask for help. This, of course, was a disaster. The committee simply condemned the picture. They had no reference for it and we had no history.

We did find some suitable examples of handwriting over the next few years and in 1990 the expert wrote to us saying that: "There are some fairly good matches between the writings but I keep coming back to the letter 'd' " - she was unable to find a match for this letter in the same form. She continued to be encouraging and suggested that we keep searching for painted handwriting.

I suppose we did make some effort to find more reference, but not a lot. We were always busy with other things. Mike made a very nice box for the painting and for the next ten years it rarely saw the light of day. I haven't seen it since the year before Mike died. I don't even know where it is. I miss my friend a lot more than I miss the painting - I'd rather hear Mike's voice on the end of the phone with a cheery - "'ello, mate. You 'eard the one about the bow-legged vicar and the policewoman?" - than ever have a provenance for a painting - even Picasso's Guernica in colour.