Thanks for pointing me in the direction of that great little Add In. It works perfectly for what I am needing to to (in this case it is identify the areas of sea that are surveyed from given land-based surey sites). There wasn't even too much fiddling around to get the exact outputs that I needed. I can see it being used quite a lot by ecologists (my field of study), even though its designed for working with Wireless coverages, as this is the type of thing we need to do quite often.
All the best,