I reply to most things straight away, so it goes into the diary and it's "done" - I just need to turn up with a gift and a smile

That being said, I have heard lots of negative feedback from all different quadrants lately about seriously crappola attitutes to RSVPing lately - for everything from parties to group events to weddings even!! Eg: a girlfriend recently had to ring 10 or so guests to her wedding (including two cousins!!) to find out whether people were coming or not, because she had to finalise the seating charts for the reception venue. The invitations had gone out almost 3 months in advance, and 2 weeks before the wedding, she was still waiting on a bunch of replies ...

And I've heard very similar things for kids parties, housewarmings, and all sorts of other invitation based events ...

I think it's really rude, but it sounds like you're experiencing the same thing ...

And no, I don't think 10am is a silly time. I actually think it's better than a 12noon thing, because the family can still have the afternoon free for their usual activities