Thanks for your reply. I measured the battery voltage in place as you suggested. With the unit switched off it measured 3.0V on the nose, but powered up this dropped to 1.88V. Returned to 3.0V when switched off. Is that normal?
Also, there is a socketed 8-pin DIP chip to the left of the battery holder – U1002 – which I took out and forgot to replace before powering the unit up. This appeared to make absolutely no difference to the boot loop issue. I don’t know the purpose or type of the chip (my eyesight isn’t good enough to read off) – would it be worth replacing it?