Ah well, disaster.
I bought a replacement 2nd hand. I installed it fairly easily.
Tested it on setting 1 - fine
Tested it on setting 2 - good, air coming from the right places.
Tested it on setting 3 - even better, lots of air coming.
Onto setting 4 - noise as if the fan was catching.
Took it apart, couldnt see anything. Couldnt see anything in the drum.
Back together again. Same tests.
Then on setting 4 -CRASH! and the fan blade fell apart.
I cant work out what happened. The plastic cover on the bottom has a small crack in it, but I cant think that caused it. Must be a problem with the motor or blade spinning off centre on high speeds.