I thought I'd finished editing my video and exported the MP4 file, but after watching it through, I spotted a problem. One of the cubes apparently teleported!
I had edited the video to remove some hesitation from the audio and hadn't noticed that I'd moved a cube during the part that got cut. So I re-edited that section so you can now see my finger moving the cube and I exported the MP4 file again. Watching it through this morning I realised that, somehow, while correcting one problem I had managed to accidentally delete the background music for the Scrabble section.
So I added that back in, a fiddly job because I've got two video tracks and two audio tracks to keep in sync, and I exported the video for the third and what I hope is the final time. I'll watch it through tomorrow and, with luck, will be able to upload it to YouTube. Then there are still the subtitles to add, along with the video description. It turns out that shooting a video is actually the quickest and easiest part.