I recommend ditching the idea of charging your car from solar panels, just get a SEG agreement from Scottish Power who will pay more /KWHr than Octopus charge at night time. Makes life so much simpler as you can let your house export as much as possible, even when it's just a low rate due to clouds ...

Regarding the rear 12V Socket, you can move Fuse 52 to set it in a permanent ON state, (search for 12V Socket for picture posted by others elsewhere on this forum)

Great input on the regulations. I wonder what they say about reversing lights? Personally I find one reversing light somewhat useless if I'm reversing at night time. I get more light from the (red) rear lights than from the reversing lights. Assuming the regulations don't prohibit it, why can't they...

I found choosing the Cancel option to be no use, as the message re-appeared approximately 1 minute later. Carried on and after ~20 attempts at Cancel, I gave in and chose OK to make the message go away.