Many thanks to all who posted suggestions of probable causes. I was certain the fault was the alternator bushes as it was charging at only 8v.

However, the fault occured less when it was raining, suggesting an earth problem.

Problem cured by fitting new body earth strap to engine; fuel drain and charge lights behave as they should, temp gauge does not go full scale after 5 mins idling.

It does seem that tracing faults with fourtrack electrics is not easy.