First, in our Google-ready age, "Simon Gamble" is one of those especially British names. Google him - or, better yet, Facebook "Simon Gamble," and the list is long, making actually finding the right Simon Gamble a challenging task in a world of perhaps thousands of Simon Gambles.
Second, I am, of course, assuming that "Simon Gamble" is this individual's true name. Unlike today, when online monickers can take on infinite variations, using a fake name with a street address might have caused too much confusion in mail delivery. I'm betting he is actually named Simon Gamble.
Now, if I was adequately obsessive and wanted to spend the money, I suppose I could hire an investigator to explore the last known addresses I have for him in the early and mid-eighties, and trace him up to the present day - but I think that would be a going off some sort of deep end....
Picking a random letter out of the pile, written on September 9th, 1981, Simon writes about returning from a three week vacation in Greece. He'd stayed previously on Greek islands, but this was his first time on the mainland, and in Athens, in particular. He mentions that he had planned to go on to Israel to stay with relatives, but changing his plans after he "got through more than a huge amount of money" and returned home instead. He wrote that he had given his notice at his current job in Northampton (where he lived on "Shakespeare Road") and was hoping to find a job in London and move there. In the meantime, he would be living off of his savings. He doesn't indicate what the job was - but perhaps other letters will provide clues, particularly if he advanced in that same field (though it's been 23 years since his last letter, so anything is possible).