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

Wait. Don't do that. That's too drastic. Have you got a bath full of water and inflated the offending tubes to massive then had a look at them under water? I mean really inflate a couple of those tubes so they look like beach toys. The air is coming out somewhere and the bigger you inflate it, the larger the hole. If the holes are in the same place, problem located.

Also have you really (I mean really - with a magnifier or a torch) been over the tyre / rim / rim tape with a fine tooth comb CSI style? Inside and out?

Both quicker than a re build.