I'm not an electrical engineer….just to be clear !

To reduce the work involved, take the battery of and charge it overnight, then start the car, put the fan on - does the voltage drop ? - if so clean all the connections from battery to fan, repeat for both headlamps.

If you have kept the original MGB wiring loom I would be looking at the crappy Lucas bullet connectors for corrosion, and then look at the quality of you earth cables to ensure they are good.

In summary, I think you probably have a dying battery or some really corroded connections.

Malc

Thanks for your guidance Malc however solved the problem while looking at the engine earth strap!
About five years ago I changed the engine and since then this problem arose not long after and accepted it; at the time thinking of the problem I changed the voltage stabilize to a modern solid state type, did it stop the temperature going up with the fan heater switch on; no but at least I new the engine wasn't over heating.
I fitted another engine in January as the other had a rear crank oil seal.

So, twice I've fitted an engine and never notice on both occasions I hadn't re connected the wiring loom bundle of earth cables which I had neatly tucked under the chassis strut next to the engine mount earth strap!

Plonker comes to mind Rodney!

So, all instruments working fine and the headlamps are brighter as the volts are up.