Just make sure you cater food and drinkwise for at least twice as many people than you have invited. Once the word gets around on the grapevine there will probably be lots of relatives, no matter how distant, and long lost friends who will assume they must have been invited but the invitation just got lost in the post or something.
Iain.
Didn't think I'd need to worry about sending out invites. Was assuming word of mouth! :-)