Any question answered...

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.

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.

Will give that a good go tonight, havent checked the tyre CSI stylee, gave it a good look but no to that extent and will over inflate the tube and re submerge, I only inflated them to a little over normal size last time so that might well show some sort of pattern.

Cheers guys... the plot thickens...