First of all, my thanks to all those who took the trouble to give their advice so promptly. Any delay in replying is due to my tardiness and poor keyboard skills.
The upshot is that I abandoned my original plan and looked on-line for an alternative. A phone call to the nice man at ffordes Photographic in Inverness showed that a Nippon Kogaku Variable Viewfinder ( right vintage, decent condition! ) was available and it is now on on its way to me courtesy of Royal Mail.
Again, thanks for all your advice
Colin