I'm having some difficulty charging the Mini, running the original (I think) generator. But the battery doesn't seem to charge while driving.
The IGN light on the speedo has been replaced twice by me, every new one lasts until I rev the engine, where it glows really brightly and then pops (blown). I'm using 2.2w globes (not LED's) in there.
Is there a simple test I can do with a multimeter to determine whether the generator is working, or where the power is not getting through?

At present I'm using a trickle charger about once a week to keep the battery topped up. My own plug in hybrid Mini.

How old is your battery?

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2 years - but I've only really been using it in the past month or 2.

it's not a problem with the battery, it's a problem with the voltage regulator - the IGN light is connected to 12v on one side and the voltage regulator on the other. When the generator isn't charging, the side that goes to the voltage regulator has 0v on it, current flows so the light comes on, when the generator is charging, the side that goes to the voltage regulator has 12v on it, current doesn't flow so the light is off. What I think is happening here is that the side to the voltage regulator is seeing way more than 12v, even up to 30v, and the light is blowing... I'd check the wiring to the voltage regulator, you might just be hooked up to the wrong terminals

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I'll check the connections against the wiring diagram this afternoon, but I suspect you're right - and this would probably stem from me ordering the wrong wiring loom!
(From memory I had trouble locating wires of the colours that were mentioned on the wiring diagram).
Ill get another globe as well - hopefully the last!

Not good for a battery...I would think about a load test...

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Have a look at "Charge Light Stays On" thread started by haze blue 850 in August 2020. He/she had similar issues.
If the Voltage regulator section of the Control Box is not set correctly then voltage output of the generator (at D) can just keep rising as Simon K suggests. As well as frying the light it, won't be good for the battery and any modern electronics in the car.
Check that you have the right leads going to D, A, A1, F and E. Take the cover off the Control Box and have a look at the voltage regulator contacts when you start the car. They should be sparking, like ignition points. Don't just fiddle with the adjustment screws.
Follow the complete procedure in your manual in order, with an ANALOGUE multimeter. Good Luck Dave D

Check the earth on Regulator unit Marked E.

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Ok that was easy - E wasn't connected to anything. Thanks Convertible Mini!
All the other cables checked out fine.
Now the light is on when the battery is on, but goes off soon after starting.
Voltage at idle is 14-ish (got to tune the idle back down again)