Hooray, the ‘o’ key is working as normal again - life is good :)
Had to have another visit to Vision Express today - one of the lenses was worryingly loose. I had a good look, and could see that the loose lens had a plastic washer where it was screwed to the frame, whereas the other one had a metal one. It made sense that the plastic one would deform, and so would loosen - they cannibalised another pair of rimless glasses on display to get another washer, but they seem ok now. Hopefully that will be the last time I have to visit Vision Express!