Hi sammo440,
I have done courses with diane lee and LJ lee (who are unrelated) and i know they keep a register of people who have done their courses. check out Discover Physio - Diane Lee and LJ Lee and try to contact one of them - what you want to find out is who has done "The Series" - these are physios who are trained to think about the effects of seemingly different areas and how your neck and affect your knee with a logical explanation.
I have fixed many a sore knee with back treatment - i have fixed many a sore knee with knee treatment too!
Stretching for tight muscles is one thing but you can *feel* tight but actually be "overactive" which is a neurological thing, not a physical shortness of your muscles.
Anyway, i would be happy to put money on it NOT being your knee at this stage - please remember that it is very hard to diagnose and treat online - nothing replaces actually having a physical assessment - which is why i am skeptical of promised internet consultations!
Cheers