I had a session with the sports therapist/torturer today. As I thought it's the ITB that is the problem - turns out that both legs are bad, though only one has been giving me trouble.
And ST has given me a whole new set of exercises to do, that put less pressure on the knee but will strengthen my quads and also my core (groan).
She is ordering me a ITB roller thing - meanwhile suggests I use a rolling pin (not to lie on but to rill up and down - like the stick) and lending me a wobble board - when someone returns one that has been lent!
I am seeing her again in 3 weeks, when she will check up how much progress I have made - so I'd better do them!