While I'd be pleased to forget about all this, I think it's clear now what happened.
I've remembered that the previous evening, I managed to knock the bins off the coffee table next to my armchair (where they are usually left readily to hand) into an awkward gap. Just a 15 inch fall on to thick carpet, no possible harm. But I have balance problems, and rather than stand up and get past obstacles to fish the bins out, I almost certainly just reached back and picked them up with one hand - I can only suppose, by the focus wheel. Having tried it just now, what of course happens is that the dioptre adjustment mechanism is exposed. Unbelievable, I know.
The following day, I must have failed to notice this(!) and started trying to use the bins normally. Ted said: "Didn't think about it before Lee, but if the Swaro focus is pulled out to set the right diopter, then the travel is limited to +-4 diopter of adjustment, or just 1\2 turn of the focus wheel!" and that's exactly it, though the range of adjustment is +5 to -5, which is indeed very nearly a half turn of the wheel. The "grating" sound would be the usual clicks of the dioptre adjuster in rapid succession.
All of which I think people have already concluded! I can only say I hadn't been thinking too clearly (a known side-effect of a pain med I'd been put on. With doctor's agreement, I've reduced the dose).
Anyway the bins are still in good shape, and stiction-free as well. Only thing I'd add is that, being hypersensitive to this, I've been very carefully checking the focus action. If the wheel is turned clockwise to its full extent, then anticlockwise again, when resting a finger lightly on the wheel there's a little play present, though it's very slight. But immediately after this, when turning the wheel again, the play is absent and focus is immediately responsive. Seems likely this is just the way spring in Swaro focusers work, and was probably there all along - it's as if there's slight temporary "slack" which disappears when taken up by the action of the spring. In any case, no reason to send them to Absam.
Excuse the length of this screed, and thanks once again to all who have responded and made suggestions.