Hey folks out there in spanner land - I found the fault myself!!
While waiting for somebody to have a think about the problem I had another look at the thing and a few other websites and found what seems to be the answer.
If anybody else has a similar problem it is probably a fault in the motor tachometer. This sits on the front of the motor and is retained in a housing by a big star retainer approx 30 mm dia. I found the star retainer lying on the floor, took out the tachometer, screwed in the rotating magnet as this had come unscrewed from the end of the motor shaft and put the tacho back and retained it with the star retainer - after straightening out the tabs a bit so that it had a bit more grip. Now it seems to be working OK - cross fingers; touch wood etc.
Glad it was a basic mechanical fault as without a wiring diagram and some indication of how the system is supposed to work, it would have been extremely difficult if it was an electrical fault.
Keep on repairing them - don't add to landfill just because of the greed of the manufacturers.
Revy Trevy
Trevor
March 2008
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