I don't use heat mats at all; I put my seeded flats in a warm room until things start sprouting, then they go onto my (homemade) lighted shelves, which are in quite a cool area of the house so I wrap a sheet of plastic around the whole setup. Nick, you might want to give that a try - the plastic holds in the warmth put out by the grow lights, as well as some humidity. (The plastic is pulled back for the photo; it normally wraps completely around the whole unit.)
