Thank you NightQuilter for taking an interest. I think it's good advice about exercising the quadriceps though I won't be doing any more step-ups - at least for the time being.
However, I noticed the instability starting while I was still doing quadriceps exercises (inc step-ups). The persistent swelling, heat and pain (the latter two managed by icing) I think mean there is some underlying problem e.g. osteolysis. The further tests will have to wait until the covid-19 storm blows over. Meanwhile, in the words of the surgeon, I will have to "tough it out".