Right, may have some progress here.

Scud not arrived yet, Royal mail tracking just says "on it's way" - No ****?

Inspected the in-car cable [supposed genuine nokia cable from ebay] and noticed the plastic mouldings were rather "shoddy" looking, like they'd been carved by hand. So I got another cable. No charging issues in the car for 3 days so far.

Turned the phone off last night, and it turned on OK this morning. Might be coincidence, or perhaps the cable was affecting the phone's charging when on the wall plug etc.

I contacted the ebay seller and they sent me another cable, which looks much more genuine than the last.

Will keep you updated of the scud, and I will be turning the phone off every night from now on to see if it continues.

Cheers for all the help.