Gimpy. I hope you get over it mate. I had about the same problem as you. Picture the Knee cap as an " O ", my pain was around " U ". When I sat down for long periods of time, I would have to stand up, and bend my knee a few times to let it crack, to release some pain and help it. I took about 3 weeks off, and it's about 90% better mate. I went to a physiotherapist and it was diagnosed as " Runners Knee ", medically known as Patellofemoral Pain Syndrome. They advised for me to check my shoes and make sure I have the proper pair, stability , neutral, or mobility, depending on your arch, or any pronation issues. It can also stem from weak inside quad muscles.
Some of the exercises I did were the following - I did a lot of swimming, and running in water. Avoid bearing a lot of weight on the knee, such as squats, or leg press. Try lying down on your back and do leg lifts, try to keep your leg straight, and if you have any, try some ankle weights. Although you are suppose to avoid squats, you can put your back against the wall, put your GOOD leg in front of you, and go down half way and back up. These are just half squats and should not put a lot of weight on the knee.
In no way am I a doctor, or neither is my dad

, but this is what just helped me. Look over the link below, and see if this sounds like what you might have. I still advise you seek a professional opinion.
http://www.drpribut.com/sports/spknees.html
Crack on mate !!.