Progress so far:
I read a few suggestions about tacho magnets falling off the motor but the magnet and pickup (200 ohms and 400mV when the drum rotates by hand) seemed ok. I traced the tacho route onto the board. The pulse 'squaring' transistor (marked Fp1) was suspect C-E short. The transistor was desoldered but tested ok on its own.
There are a few links marked 'Ground' and '12V' so I powered these up up from a battery and checked the 5V regulator was ok. The tacho pulses were reaching the square NEC chip at the end of the board - I think this is the brains of the outfit and probably where the problem lies.
Anyway I could not find anything obviously wrong so I put it all back together and the damn machine starting working properly!
I am about to try a full wash and give it a test.
Graham Thomson, July 2007