I feel like a neglectful parent. Whilst polishing my entry for Spring Thing, I avoided updating to 5.4 because I knew it would be very different, I was afraid I would break everything if I tried to do that.
Well, as it happens, I loaded the game up today to test it in the web browser and see if it had started working online (it hasn't, really) when I noticed that the background image hadn't loaded.
Upon testing the original file I had created and uploaded here and to the Spring thing website, I noticed that the background image wasn't displaying at all. Just to check, I re-installed Quest 5.3, and it worked as always. So since the start of that competition, my game hasn't been displaying properly on the most likely version of Quest to be downloaded, and the only person who could have spotted that problem was me.

In short: I think there may be an inconsistency with how Quest 5.4 displays background images in older versions which is causing it to not function properly. Luckily, this is only an aesthetic problem, so no real harm done.

Thanks, I've logged this here https://quest.codeplex.com/workitem/1276 so will take a look into it.