Thanks for responses.
I checked the seat tube very carefully- no evidence of any 531 or other transfer there. No evidence of a name on the top tube. I don't think it has ever had any.
Looking at the Viking site, the Conquest seems very similar, except this doesn't have a lamp lug on the forks. It had a centrepull hanger on the headset.
I have taken the forks off in case there was a number on the stem- nothing!
My daughter has helped re-read the frame number. Could be:
599 306 9022 E38
The other number on the rear lug could be 217 It only seems to say "YCLO", but the C could hace been brazed over.
I weighed the frame and forks at 5lb 10ozs. The frame alone was 4lb 8oz. Seems to compare favourably with my Claud Butler which is butted tubes, forks and stays.
I have uploaded some more detailed photos of the front drop outs which are nice, and the frame has a brazed rear gear change, with pump lugs on the top tube.
It's rusty but true, with a slight ding in the top tube. It would paint up well, but need to be done carefully, so it would be nice to know its age. I'll put it back together anyway and see how it rides, but it may be too small for me.