There were only a couple of pieces I didn't recognise and would want to buy. These were in the final part, after the speeches, and most online soundtrack lists seem only to show what was played before that point. I would like to know everything that's on the album before shelling out for it.
Judging by the YouTube upload, Underworld's epic piece for the Pandemonium/Industrial Revolution sequence is the studio verison... and it's magnificent! As Adamsim has said, it's titled "And I will kiss". I've also listened to most of the tracks uploaded elsewhere, and they were all studio/original versions.
I believe you're talking about the music in the part of the ceremony where Steve Redgrave handed the torch over to the first of the young sportsmen and sportswomen, continuing up to and with the lighting of the cauldron (code named "Betty" by the designer and shown in stylised form on the CD cover). That track is "Caliban's Dream", which is on the CD (final track on disc 1).
The other major new work was the musical backdrop to the Pandemonium scene with all the drummers. That's "And I Will Kiss" (fourth track on disc 1).
I can't find the references now, but I'm sure I read online that all the new material was recorded in a studio session at Abbey Road Studios - though I've also read reports that some of this material was recorded at the Olympic Stadium (and presumably taken to Abbey Road Studios for post-production). I can't think they moved that huge bell to Abbey Road!
The tracks on disc 1 that have been previously released as recordings seem to be previously released recordings (such as Dizzee Rascal's "Bonkers" - 2:56 on this CD, 2:56 on the "Bonkers" EP; Emeli Sande's "Heaven" - 4:12 on this CD, 4:12 on Emeli's "Our Version of Events" album), even if there were some differences between the recording and the live version on the night. "Chariots of Fire" is a new recording of the 'Mr Bean' version.
One thing is certain - none of this material was recorded live at the Opening Ceremony. There wasn't enough time for post-production between the ceremony and the material becoming available to download almost immediately afterwards on iTunes.
If you want to check out the majority of new tracks, the double album is on Spotify (and should be playable once with a free account), though there's quite a few tracks missing there, presumably for rights-related reasons.