Right leg seems to have improved after my initial sharp pain at the start of wk4. Thanks so much to everyone for your help and suggestions.
The left leg calf pain seems to be a sort of heavy cramp in that it 'moves about' as I move it. It's an odd feeling but it kicked off again this morning about half way through session 2. I sort of 'breathed into it', as they say in Yoga, and kept gently moving. It seemed to help.
I shall keep with wk 1 and gently work it through. Hopefully back to wk 4 next week!
Also, in case anyone is looking for help on a similar mid-foot running problem, I found these great calf stretches on YT: youtube.com/watch?v=Wgv9nq-...
Keep going, everyone. The goal's in sight!