Success! Brasso worked eventually!
I did more rubbing with Brasso. Superficially I put a lot of radial scratches on the disc, but eventually I gave up, rinsed it under the tap, dried it off. Then put it back into my iMac and waited. This time iTunes seems to have ripped the whole thing, and interestingly the time for each track seemed very much shorter. I'm guessing that the CD may still be difficult to play, but now I've digitised it - hopefully accurately - I should be able to play the files or make a CD copy.
It was rather tedious nevertheless, but I thought it was worth trying to rescue this one - Naxos 8.550358 Idil Biret Chopin Mazurkas - the things I'll do to avoid having to pay out more money for a new copy!