That was the general opinion of the physiotherapist. The movement of pain is apparently quite common and tends to show compensation between muscle groups and ligaments as they each try to help each other out. The main issue seemed to be marked tension extending from my lower back all the way to the sole of my left foot. This is putting my pelvis out of balance and so every step requires a twist of my leg through The knee.
Forty minutes of massage and it seems I'm a little more balanced and I can feel a reduction in tension through my calf muscles.
The original cause is unclear; probably a gradual fatigue and spasm with a 'final straw' after the bank holiday marathon and the new-shoe-run. The good news is that as there is hopefully no underlying injury I might well be fit in a month or so, Until then, I need to try short runs and see what happens, and of course, keep up the stretches.
Another interesting point was raised, "What about your walking?". On average I walk 30 miles a week just through commuting and more if I go on a hike. As I've done this since I was 18 it doesn't feel like training. However, this is wrong, it makes great base training and whilst I'm doing little running it is still wear on the legs. I probably should log this mileage and consider it part of my training plan, especially on weeks where I do a longer than usual walk.
Swimming is allowed too, so now I have to work on not bobbing my head side to side since my arms are getting tidier.