Bill, I don't know if you remember Mike Nicks, but you're right, it definitely isn't him on the bike! I once remember doing a test with Mike on Cheney Triumph ISDT bikes, and he was no great rider. Bloomin' good journalist though.
I am amazed that no one has responded to your querey. Surely someone will recognise him? In my opinion, it will be a MGP rider who retired in the late 60's early 70's. Ken Huggett?
Just a stab in the dark, 'cos I've no idea.