Just in case anyone in the future is interested, Ian May Cycles can still be found at 170 Bebington Road, Bebington, Wirral, Merseyside, CH63 7NX.
I've dropped them a line to see if they can give me any information on its "pedigree" (and supply decals for the respray).
Yes, it is a twenty-three and a half inch frame (centre of crank to top of seat tube) and I am six foot three inches tall.
Thanks for your interest.