Just an update for anyone suffering the same issue as my MK V GTI - aircon has been taking 10 minutes at least to start after pressing the button. Originally dealer said it needed re-gassing, had it done and no difference, last time they thought it might be the pressure sensor but then decided it wasn't - now it's in for an overnighter and the compressor is to be replaced, I'll post an update when I get it back !!
First VW I've owned and the the first car I've had to take back 4 times for warranty work

At least its nothing serious though