A helpful soul (the co-founder of chipmunka) suggested the following as an alternative: he would write a fake letter, which he would then claim was by the author's relative (me)!
He provided me with a sample he had knocked up himself:
"I am just sorry that I acted in a strange way to you. I am sorry that I left you feeling like you, like I disowned you. I had not realised the pain this caused. I am very sorry about my behaviour"
Aside from making it look like I am unable to write coherently, this did not reflect my feelings then or at any time before or since, and I certainly would not have written anything like this.
I tried to explain this, but Chipmunka felt that as my name had been changed, it didn't matter what they said. In fact, I was told that they could publish anything they liked and I would have no way to fight it:
"Basically the character doesnt have your name and as therefore it isn't you we could have this character say whetever we like, legally."
I guess if there were a 'character' rather than a thinly disguised version of me in an autobiography there would be no problem, because I am not a fictional character. Chipmunka's shaky grip on reality seems to extend to the existence of people they correspond with, unfortunately.
I did let them know (in a non-fictional way) that this was not acceptable.