My original reply suggested the frame was late 70s.
Let's not confuse Carlton with the SBDU. Carlton Worksop bought by Raleigh in 1960, Worksop factory closed in 1981.
SBDU only built made to measure, no off-the-peg frames. Created in 1974 by Gerald O'Donovan in Ilkeston.
27.2mm doesn't guarantee 531 (it could be 531 and 27mm - there are butted and plain guage tubes in 531) and then there are other 531s - 531C, 531SL, 531Pro etc.
531 is a classic and superb tubeset, but a lot of mass produced frames and bikes in order to benefit from the halo of 531 often had only 531 main tubes: unless the sticker says"main tubes, stays and forks", then it only has 3 main tubes of 531 and then often only plain guage ones.
I ordered a new SBDU frame in the day (still have it) - a graduation present from my Dad and have two early Carltons (all 531 throughout).
This Carlton is good frame and should be preserved and used, but it is not an"exotic".
Hope this helps, cheers, Roadking.
I wasn't confusing Carlton and SBDU, however, SBDU made a few off the peg frames, but not many.
I only said 27.2 would point to 531 - obviously other seatpost sizes could indicate quality tubing, but of course the smaller the seatpost, often the cheaper the tubing, at least with older road bikes. As I understand it, some makers reamed out the seat tube a little to suggest a better tubing than a bike may have had in reality. The age of this bike would point to it being before 531SL/C/Pro etc I believe?
I agree about 3 main tubes 531, although it indicates a frame of at least reasonable quality, the lugs, fork crown, dropouts etc are as good as pointers to frame quality.
A Worksop Carlton is not an exotic, you're right, but I'm don't think I was saying it was. There are very few real exotics anyway, although personally I'd class your SBDU frame as one. However, I would consider all of the Worksop built bikes to be worth a respray, if the OP likes the bike.
You are a lucky man (with a generous dad), I wish I had an SBDU frame!, especially a 753 Team livery frame - one of my dream bikes.