3 possibilities-
1. Fan belt loose...
or-
2. Alternator pulley very badly worn (the steel ones wear U-shaped, then the belt slips).
or-
3. Alternator's brushes are on the way out. Easy fix on most Lucas alternators as their brushbox is external, on back of alternator.

If it's charging properly, the voltage measured across the battery, with lights off and motor doing 2000 rpm or more, should be 14.0V or better.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Mate, I'd hate to think what she would say if you were to stop living up to her expectations in the bedroom

Back on topic, what the doc said plus:

4. Regulator shot, not supplying field voltage to the alternator.

I had this happen. Bought a cheap external job to replace.

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"Life is not like a box of chocolates... it's more like a jar of jalapenos. Whatever you do today, may burn your arse tomorrow."

Like all married men, I must say that expectations in the bedroom are a very touchy issue on either side of the relationship.

After a quick check in the dark I think the problem is one of many oil leaks spraying on to the fan belt helping it slip.

I can easily move the belt on the alternator wheel with one hand and I feel the presence of oil.

Will muck around with it tomorrow.