I've got some audi alloys, with crap cheap tyres, which I can pump up, and they're fine for 3 or 4 weeks, then relieve themselves to sort of 15psi, then stay there.
Wheels look in great condition, no corrosion (1 has a bit on a spoke, but nowhere else) tyres aren't brilliant, but I can't imagine they leak or they'd be flat pretty quick

There any kind of sealer I can spray inside the valve (removing the valve core) to try and seal the tyre and wheel?
I don't really want to have to strip them all off or anything

Try just changing the valve inserts as the rubber o ring dries out, easy to do you can get the valves in packs of four and tool (has a valves seat refacing tool on it also) from halfords, all the goo I've seen sold to seal tyres up looks a right mess to deal with when getting a tyre change

[Edited on 15/6/13 by Mr Whippy]

Good call! I'll go get a pack of valves now. The valves have never had any caps on them since I got the car 2 months ago, and however long before, so I guess they're prime suspect!