Cheers for the replies, The Mk2's battery is only 18 months old, so is been replaced under warranty. I think I'm just going to disconnect that one, then give it a couple of days on the trickle charger every other week or so.
Last winter the one in the Mk6 went flat after about 2 months of been sat, but started fine using jump leads. I did get all sorts of warning lights flashing on the dash, but they all went out after a short drive.
Both cars are garaged in integeral garages, so they don't get too cold. So hopefully from now on, all will be fine.