One solution to the amount of cheese you require is to use parmesan as well as cheddar, reducing the amount of cheddar (and thus total fat) with the parmesan increasing the cheesy flavour. Course you could also use Grana Padano instead of parmesan (Grana Padano has 1% carbs whereas Parmesan is 4%). Anyway you need a lot less cheese this way. If you can make these cheese sauce without flour let me know. (I haven't tried Mary Berry's cheat cheese sauce:
http://www.bbc.co.uk/food/recipes/maryberryscheatschee_73830, according to my recipe program it's 42g carbs, which is more than a standard recipe (25g butter, 25g flour, 1pint milk, 80 gram cheddar)