Only just saw this post.
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About a month later, I tried again, and touch wood, so far, no pain. No idea what caused it - my knee wasn't noticeably swollen, and as I said, whenever I had pain it was only to do with bending the knee - whether I put weight on it or not made no difference: I could have done ten squats holding nothing, or holding heavy weights and the pain would have been the same.
It seems back to normal at the moment, and I'm trying to regain the lost fitness, and lose the gained pounds! I wasn't very active on here whilst I was off as for me this site is very connected to my cycling - sorry for not noticing sooner!