Might find not using the flexi-mat and using the fixed mat system an easier otion mate
Need to have 30mA protection too mate depends how ropey your sparky was lol.
160w per square meter tiles, 130w per square meter wood flooring.
might want to use the 10mm insulation boards, I always do but they ain't cheep circa £20 per square meter.
Hope it helps, speak soon, lol you might know a cheep source