Here's what I did:
I used Womble to fix the GOP errors, which left me with a copy which was already IVTC'd(!) So I simply told Avisynth to assume that the 30i clip was 24p, because it was... up till a point.
From the opening logo to the opening title, this worked, but midway through the first shot, interlacing showed up. So I made another copy of the film, this time IVTC'd to 24p to kill the interlacing, and found the exact spot where the "switch" occured. Then I cut the two clips together.
Fortunately, I won't have to do a lot of this because I only need a half-dozen shots, but it's going to come down to eyeballing it to see which IVTC (ShowFiveVersions) works for each shot, or whether the shot is inexplicably already IVTC'd, or a combination of both!
Argh!
Sorry I thought it was a PAL thing, my bad.