Screw terminals can work loose over time. I had to do exploratory surgery in the kitchen ceiling a few years ago when the lights started flickering occasionally --- the old junction box was the culprit.
The job is now done. In the end I did use the Toolstation lever connectors: the positive reviews were at least as numerous (if not more so) than the negatives, and the spring closure seemed perfectly OK to me: with all my effort I couldn't pull out the wires, and it's hard to imagine them being subjected to a greater force, given that the cables are tethered and the whole thing is behind a blanking plate. I don't know about the rating, but I suppose time will tell.
I was also able to get a proper horizontal run, and marked the blue wires with red insulation tape (not having any brown).
Many thanks to all for the help and advice.
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