A lot depends upon something called MOO. Which means that they are obliged to come to your party and you are obliged to go to theirs.
If its sunny and your party is under an umbrella, that also has something to do with it, but I'm not sure what. Parasol might be able to shed some light on that one, or not.
Remember that even if no one turns up, the IR can 'deem' that all 60 were there and so you will still have to pay up.