Any question answered...

It's an old tyre but hasn't been used and the rubber is in good condition etc, the tubes have no obvious holes that I can detect from submerging them in water the next day. I guess I am going to have to just strip the wheel completely and start again and see if that works

They must have holes somewhere otherwise they wouldn't go down. Try pumping them up until they are 2 or 3 times their regular size and then look for the hole. I've had similar recently and they've been more of a scuff than what would normally be expected of a puncture. Always in a different position, always beneath a nick in the tyre and always the same slow puncture.
Vitorria Open Corsas for the record.