if I were to use chisels to route out the body, say its for a humbucker and I chisel it out 2mm smaller than it would normally be, then sand it flat., would I get away with it?
It doesn't really matter how you do it, so long as you're careful you could pretty much do it with a spoon.

Be sure to measure accurately and take your time, you'll be fine.
thanks, and I think I will stick with my chisels although I have plenty of spoons!!!