Guess who's got too much time on his hands?
I was wondering if you could date it from the stem. Older GB stems seem to look like the one on your bike, with the horizontal bit continuing past the steerer, rather than being smoothed off at the top. Maybe it's a GB Spearpoint stem?
I think a combination of the shape of the rear dropouts, that unusual chainwheel, the stem, anything stamped on the hubs or brakes, plus of course the frame number (see
here for a guide to Elswick Hopper frame numbering) should sort things out.
Now, a life. I must get one.