the simplest and best way is to get a very small amount of acetone, and put it in a non-leaking plastic bag. (Make it, say, 1 inch deep. Now slowly push the polystyrene into it. It melts like ice in hot water, with incredibly almost no residue. Leave the bag open when you're finished and the acetone will quickly evaporate.
I think I'll play it safe and break down the majority of it up for the binman.. I've been told with the rest that I can break it down and add it to the soil for my plants.
Neildown - you're 100% correct, my suggestion isn't environmentally friendly, but then neither is polystyrene, and sometimes it's the lesser of two evils.