Hi BritNoob, I completely agree with aefibird. Go see a doctor about your knee, just in case. But make sure you go to a sports medicine clinic. That way you'll likely avoid being told to just stop the aggravating activity (MA, in your case). My knees aren't so grand themselves, but I have a great sports doctor and physio, and my MA training has actually improved the state of my knees. It's crucial that you let your instructor know that you have bad knees, though, so that he/she can pay specific attention to correct posture and body alignment. When performed correctly karate should not hurt your body. Best of luck!