Now you've explained it Lewey a failed/failing diode is what it is. It's all coming back to me now
Our mechanic diagnosed it about 6 years (20/30k miles) back, dad removed the battery light, then when restoring the 750L I decided to replace it. I'd rather a light reminding me of a minor issue, that I can monitor by seeing if it gets brighter, than having no reminder of it
The car has sat for the last 4.5 years mind so knowing my luck it will fail now. I'm changing the timing belt next weekend so might just chuck the spare I have in it as its only an extra 5 min or so.