Could someone let me know what size caliper key I need to change the pads on my 2.0 fsi...many thanks...7mm?

I think most of the VAG range are all 7mm.

Cheers 4 that. Bit annoyed to realise that as it's easter, all the blooming shops are shut so I won't be able to get one.

I remember the good old days when you pulled a couple of pins and the nice friendly pads would fall out, that being said I plan to use a big pair of grips to push the piston back, do I need to loosen the bleed to do this or just use my uber strength?

Best to loosen the bleed nipple and top the fluid back up if required.

read that there are possible air ingress problems involved, though would imagine only if you push back the piston and forget to do it up before touching the pedal, or does the piston 'want' to close again when pushed in, i.e i force it back, and it wants to do it's own thing?, sucking a bit of air in at the same time

No possible air ingress at all as long as when you have pushed the piston back you lock the nipple off again.

Just saves forcing the fluid back up to the reservoir.