In message , Mr Pounder Esquire
writes

Can't really help, but an online friend had a similar problem. AA said
battery. Halfords thought the same but did not try to sell him a new
battery.............. hmmm.
It seems that stuff can come apart in a battery and flatten it. I did not
know this.
He bit the bullet, bought a new battery and all has been fine for weeks.

I think a new battery is the answer, but I'm really only guessing. I'm
no electrician (auto or otherwise), and apart from some logic and basic
testing, I just don't know.
--
Graeme