I was able to report to the physio that I'd done 5 short runs (with walking downhill) in the last fortnight and that most of the time I'm not aware of my knees when walking. She took that as a good sign and now I've a plan for extending the runs a little and a series of strengthening exercises. One concern is whether the runs and exercises should be done on the same day or different ones. The advantage of same day is that it allows a rest day inbetween, the disadvantage being that it could result in too much overload.
This evening's swimming focussed on a good reach - over the course of nearly 40 lengths I started to get the idea. I could tell that it was a more powerful and certainly resulted in fewer strokes per length but it was more effort. I'm back to 2 lengths at a go now.