The RSVP thing is getting really rude in society! I always think that the people you are inviting are supposed to be your 'friends' so why are they being so rude not getting back to you? I'm in 2 minds about it, either say "RSVP by X date or you won't be catered for" (or something similar like we need to know numbers) Or if anyone doesn't reply just not invite them anything ever again lol