Thanks one and all for the comments. I've just been out to check my measurements and I can confirm that the shaft thread is 22 tpi and 11/16" and measured my pals which is 20 tpi and 3/4". It beats me as to why mine would be an odd size.
I don't have access to the type of equipment to make a suitable nut, and I haven't found a listing for one, so I guess I'll simplest solution would be a replacement crankshaft, which hurts a bit having spent a lot of time truing mine.
I could be interested in yours Andy, what were you thinking, and where are you based?