I tried all sorts of things, finally changing the motor. Having done that I tinkered with the old one and discovered corrosion on the circuit board where the four pin connector plugs, in. The dilemma now is whether it now works correctly. The new one is now in situ and given the work of taking all the trim off again, I'm not prepared to reinstall the old one. I'll test it on the bench. Incidentally, I couldn't believe the variation in price from £40(which I paid) right up to £190! Now all satisfactory.
There are some very good instructional videos on YouTube.
Cheers
Jim