Thank you to Austin, Paul & Tim for your advice - it was indeed the
alternator. 6 months ago I replaced the rear bearing in the
alternator. At the time I noticed that the shaft wasn't an
interferemce fit in the new bearing but didn't have any loctite or
similar and didn't think it mattered. However, over time the bearing
being harder than the shaft, the latter had worn away. Have now
rebuilt the alternator using Araldite to hold the new bearing on to
the shaft – fingers crossed!  Guess I should have used some kind of
bearing adhesive in the first place.

Rev counter is now steady as a rock. A bit of work but £5 for a
bearing is a lot less than £169 for an exchange alternator.

Cheers

Tony