Rehab is generally progressing very well indeed. The running and walking as described previously is no problem at all and cycling, etc is continuing uninterrupted..
I'm also doing straight leg quadriceps strengthening exercises three times daily - again with no problems at all.
The one slight worry is that last night - about an hour before bedtime - my knee began aching slightly. I applied ice immediately and, as I said before, everything is absolutely fine today. But I am being careful about not bending the knee too much nor applying too much stress to the area.
Overall the first four days back running (sort of) have been successful.