In reply to Martin W:
Was in the same position last week, unfortunately at 900m on Stob Ban as the weather took a decidedly grim turn. Descent was straight out of Ministry for Silly Walks, hardly able to flex the knee on the way down, and as for the cycle back out to the van....well, let's just say I was gurning a bit.
Trip to A&E next day (could hardly move the joint at all) confirmed what I had unfortunately suspected - cartilage damage. Off back in tomo to see a consultant.
Sounds like your pain is/was similar - feeling of something "rubbing" or "catching" behind knee cap? I likened to to a bit of stuck gravel and the nurse immediately said cartilage...
Very painful to flex/feels "locked" but only in certain ranges? Same here...unfortunately at the basic range of moving it from a straight position!
I was given crutches (complete rest for two days) and a tight Tubigrip. Three days later and feeling back to normal (but no more hill days or vigorous exercise until I've seen consultant.
Cold weather somehow makes it worse (it's been going on since before Christmas, but always at a manageable level, until last week!!).
Good luck!!!!!