Finally followed advice and had a physio consultation that confirmed Dr Google's prognosis. Tibialis posterior tendonitis so 6 exercises 3 times a day and more to come in a week or so. Nice to put a name to the pain but it's going to be frustrating. I can cycle on the exercise bike so hope not to lose too much fitness but I'd rather be running.
I'll follow your posts which should help so keep running and keep achieving