You asked: "Lots of people haven't RSVP'd yet - how can I chase them up?"
Despite sending stamped RSVP cards to your guests, you will have people who stall to reply. It's normal.
The first thing to do is remember that they aren't doing it to spite you; they've just simply forgotten. Odds are they will come, they just haven't had a chance to RSVP yet.
Create a spreadsheet of those pending RSVP-ers and their contact detail. Next up: call in the troops! Get your maids or family to chase on your behalf.