In my experience with Habistat (I'm from England too, my set up sounds much like yours except I no longer use a heat mat in my big vivexotic viv) just ignore the numbers on the stat. Mine is 'set' to about 28, but my ceramic bulb is heating the hot side to 33. My cold side is at 28c at the moment because it's really hot here at the moment, I posted about it here yesterday and the general consensus was that it shouldn't matter.