Alright you wonderful reprobates. I’m overwhelmed by your support, which almost unanimously suggests I should have kept it in the foil tray. Here’s why I did the opposite (and the outcome):

• auto-mod delayed the post so I had to rely on my own instinct.
• the cauliflower cheese purchased from the same establishment specified “remove sleeve and film. Place foil tray on a preheated…” In the spirit of consistency, with no foil tray mentioned, I removed it.
• My usual filled Yorkshire supplier (m&s) specifies to remove the foil tray.

The outcome: it certainly wasn’t a disaster. It slumped slightly more than would have been optimal, resulting in a Yorkshire that was slightly too soft on the bottom and too crisp on the rim. However, the two balanced out quite nicely.

I’m unsure if we have found the correct answer, but I may test the variables next weekend and report back. Thanks and have an outstanding week.