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 =96 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 =A35 for a bearing is a lot less than =A3169 for an exchange alternator.
Cheers
Tony