Just wanted to show my first finished invitation (I just have the rest to put together now!) and hopefully this might help others that are also making invitations.
Front:
I got all the card from PDA card and craft (following recommendations on here). They are pocketfold, and I ordered pre-cut card for the main bit and the inserts. I also ordered a few sheets of extra card for the square on the front. The 'ribbon' bit is just some free vellum I got with my order, with a design printed on.
I designed all the inside with Photoshop - there are 2 inserts, one of them is double sided with directions on one side and accommodation on the other. The reply insert is perforated and guests can tear off the bottom part to post back, or RSVP by email / website.