Massive thank you to all the help and advice. It is very much appreciated.
Status Quo - car started fine this morning (-3degrees) so happy battery is OK. Charged well all journey to work. Lamp was on full for the majority of the journey (it went off for about 5 minutes).
I think I may have conflated two issues -
When driving my alternator light is very very faint, but still on. If I put on the windscreen wipers or washers it starts to glow more. However the alternator is still putting power into the battery - I have a plug in voltmeter and it's putting in around 13.8 - 14.4V. The voltage reading doesn't change when the wipers and washers are on. I replaced the alternator last year and there has been no change.
Next step: This seems likely to be a result of the manky wiring/fusebox and I'll tackle this during the easter break
In the last few days the alternator light has come on for a period of time - again, no effect on the charging, and still putting in 13.8 - 14.4V. Sometimes it turns itself off, other times, switching the ignition on and off cures the problem.
I think is down to a dodgy diode. This secondary problem started shortly after my last complete battery drain, so possibly the big recharge process has upset it.
Next step: I'm going to put the old Alternator back on at the weekend If the lamp goes off, it's a diode issue. If it stays on, then it's definitely down to the wiring.
When I first went to view the MGB, the owner said that the alternator warning light was on. I took my old Morgan alternator, swapped it and the light went out (we didn't check charging voltage at that point). I then later replaced that alternator with a new one as I thought the 'wiper glow' light was down to the alternator.
I'm just wondering if the wiring issues may be causing a failure in the warning light diode part?
