I don't know a huge amount about them, but looks like a flat head (side valve) which i think went to 1957 in production terms.
Is that your nephew sitting on it in the picture?
It looks like an R series 750cc flathead to me.
Many thanks for the info, it's a good starting point to do some research.
Yes, its my nephew Luke - lucky lad, he's just turned 29 and bought two tickets at £60 each, my brother said don't waste your money!
