Thanks to Paul Jackson & Chris Hart the car has been identified as a Diamond, built by Andy Diamond in 1976. If anybody has any further information or pictures of the Diamond cars - I believe there were only 3 - please let me know as this will greatly assist with the rebuild. The internet is a bit scarce on information! Many thanks. Steve.
Doesn't Phil Bisgrove still have the Diamond or did it sell ?
this may help too https://clubmans.org.uk/classicfiles/oldsite/Driver%20Profiles/Profiles/northen_10.htm
and this https://racecarsdirect.com/Advert/Details/85355/diamond-r6-classic-clubman