Thanks Ron, useful info, and bilc0, yep sure, although it's in bits at the moment. I'll get the frame photographed for you and post it on here.
Since posting this I've also had the head and block off it and they looks like it's a 156cc. So potentially a 160 in a 80 frame!!
The piston is pretty carbonised but it appears to have a capital 'B' stamped on it. Mean anything? I need a new ring for it.
Cheers,
Steve