I recommend an immediate knee replacement.
I've had something similar, got it first two years ago then it has been coming on and off. For me there really isn't anything I can do, just wait til it passes...
Phone NHS direct and talk to them. Ask what they think and then see if there is a doctors out of hours near you. Ours is attached to the hospital so they can send you for test straight away if they think its neccessary. In the mean time ibuproren gel (relgex is the best) and maybe some aspirin.
Surely it'd be better to make an appointment now -even if it won’t be for some time, at least its set. Especially as the problem seems to be long term, it'd be well worth getting it looked at professionally.
I know on my Mum's side of the family there have been a lot of knee-related problems and operations have had to be had etc.
I talked to my friend who's in her last year of physiotherapy, and she said that it could be that of of the muscles on either side of my knee cap is stronger pulling it to one side, and that tape and muscle strngthening exercises might help it set (pain seems to be on sides of knee cap).