Thanks for the info BrianJ - this was/is obviously a European version of thirty year old vintage, identified as a '75 version by David Silver.The frame number is 1067974, and the engine number is CB400F: 1068776.
Seems my 'bike is the only one around here - I put it into the shop to have a screwed-up stud removed, and immediately had 4 offers made on it.
Thanks again,
Terry (the one in Buckinghamshire, UK., not the one in Oz)!