That's a difficult one. You mentioned the frame had been repaired... is that at the head-tube? It looks a bit suspect, and clumsily painted. If the seatpost is 26.2 that suggests the frame is not quality lightweight tubing, but I'd expect a frame with forged dropouts to be good quality tubing. Are you sure it's not 27.2? Given your location and the Zeus evidence, I guess this frame to be imported from Spain? Don't get to see many Spanish bikes in the U.K. so it's difficult to judge... The components seem to be late 60s/early 70s. I cannot tell if the frame is similar vintage. The forks have an attractive curve which also suggests quality. See if there is a number on the steerer matching the BB number.
Did you weigh the frameset with, or without the bottom bracket? Either way it's lighter than my 531db, although a smaller size, (and lugless).. Do the seatstays taper towards the bottom, or are they constant diameter?
I still believe this frame to have been intended for sprint/700c wheels. From what I can see you might still have trouble getting a 27" wheel in there even with narrower tyres.