It depends how thick the slice was cut. If it wasn't enough then you can't really class it as a HE and if it's too much you'd have to add syns to it. If I were you i would invest in a cheap set of kitchen scales. You are allowed 42g of reduced fat cheese, I'm guessing that a 'normal' slice of cheese wouldn't be 42g.
I agree with that completely, much better to go under than over.
I would still invest in a set of scales though so that you can make sure that you are getting the daily requirements for a balanced healthy diet. You'll also need it for cereal and other things too.
and you can grate it and put it into the freezer - so you don't feel tempted - great idea for saving money too when it's on the cheaper side maybe....................