I have fettled bodged the tumble dryer back to life.
The parts arrived last week (a set of carbon brushes which contact onto a couple of conductive bands on the outside of the drum ), so I set about replacing them this evening.
At the same time, I decided to track down the mechanical ka-DUNK ka-DUNK that happened every revolution of the drum, which was very loud.
Sounded like the drum was hitting something.
Turned out to be simple enough, but took a lot of stripping down to get to the problem.
The drum runs on a bearing at the rear, and the front of the drum sits on a pair of idler wheels about 4" in diameter, which run in a wheel track on the outside of the drum.
The wheel track on the drum had a couple of big deposits of hard-baked eek on it, which went over the wheels with a ka-DUNK.
Chipping this off and cleaning up the wheel track restored smooth and silent rotation.