Don't know if you've had much joy with Dennis Leech info yet but I can give you further info about his past racing.
You mention the BTCC Sierra, well that was very much a self run op using an ex RS500 shell and an engine prepared by a firm called Mass or was it Empress?
Prior to that he was well known for his exploits as a privateer in the BTCC. In the 70's he raced Camaros/Mustangs and then Capris and then latterly Rover SD1's between 1982 (the last season of Group 1) and 1988, when he had a nasty shunt at Thruxton.
There is some reference to his Rover exploits in this thread:
http://tentenths.com/forum/showthread.php?t=81326
Dennis ran a black with Dayglo orange stipes colour scheme on these cars originally made famous in UK by Dave Brodie.
I think legend has it that Dennis bought a Grp 1 Capri from Dave in the late 70's and then carried on with the same livery as his own kind of 'signature.'