Squealing belts? but only when lights, heating and windscreen wipers are all go

Just to explain what is happening for future reference, the more current you need to run your lights, heater, wipers etc. the more the alternator has to 'work' the more it 'works' the harder it is to rotate as it's generating the electricity. Normally what happens is that as you start the car the starter motor takes a chunk of energy out of the battery which the alternator tries to replace, hence why some belts will only squeal for a few minutes until the battery has recovered. I suspect yours has now got past that stage & is slipping even after the that point, it might be worth in the short term until you can look at it, start the car, give it a minute before driving off.

Also check for a slight coolant leak.  Leaks from the top hose or the pump tend to get onto the belts, and it's slippery stuff.

Thanks for that description. That does make a lot of sense in my Jimmy