Thanks guys, and especially Shooter. A Linear wipe did the trick nicely. It is not 100% because the wipe obviously reveals the film strip, as opposed to the film's leader travelling across the screen, but this takes place over just 12 frames so I doubt anyone is going to notice.
I am also noticing some flicker with the large jumps between frames, although I am viewing the output on a computer monitor. Frame blend is not really helping, so for the moment, I have added motion blur to the canister and film strip for the frames they are in motion. This has fixed the flicker, but if there is a beter way I'd be pleased to hear it.
The whole video editing thing is proving a slow learn for me, because I just don't enough of it. Having a good knowledge of Photoshop is helping, though.
Thanks again for the help.