Here is my recipe for microwave cheese sauce.
Take one teaspoon-sized blob of margerine (the sort you can bake with), a tablespoon of plain flour and half a pint of milk. Stir together and place in the microwave on full power for 2 mins (I use my 1 pint pyrex jug for this).
Whisk well to remove any lumps, then add the grated cheese (200g or so, the quantity isn’t critical) and whisk hard again, try to dissolve the cheese as it melts. Add another half pint of milk and some black pepper, stir well and leave to stand.
Once you are ready for the cheese sauce, pop it back in the microwave for another minute on full power, then serve