Sudden deterioration of print quality on Prusa i3MK3S+
Figured it out. One of the grub screws that hold the motor shaft onto the belt drive pulley for the Y-axis had come loose, and the other had fallen out altogether. I've tightened the remaining one (and ordered replacements), and my prints are perfect again. Well, as good as I can expect, given that I'm using cheap filament.
Two points that might be useful to bear in mind; firstly, the printer was consistently reporting that the belt tension was fine, but clearly it was not. I'm not sure if this is an issue with these printers in general or mine in particular. Secondly, the printer was able to figure out that the pulley was slipping, but did not do so as part of its standard test/setup wizard. It's under Calibration &gt; Self Test.