put your foot down and zilch, the revs just move up very slowly
That's pretty much it. It kind of happened gradually over a couple of days, so hopefully a good clean will work.
In lower gears especially once the revs are around 2500 to 3000 it's fine, just at low revs. The engine is now very quiet and smooth following the cam belt problem, and there is no smoke from the exhaust, so I don't think it's a timing issue.