Physio identified it as a tendon. Took ages. Complicated things knees. Gave me lots of stretches to do.
I think the week off 'Bromptoning' clinches it though. Immediate improvement. Immediate Northern Line cold though
Running feels quite good at the moment so knee can't be that bad. Perhaps I should run to work!
Anyway, I've ordered a new seat tube. I'll probably fall off at the first traffic light!