I used to have a very similar problem. It's a really uncomfortable pain? Feels like somethign just "isn't right"
I went to the docs who suggested my knee cap might be slipping so sent me for physio.
Physio just said it was somethign to do with the muscle not being stretched properly and rubbing against somethign or other.
Point is, he gave me some specific stretches to do, especially before exercise, and it pretty much cleared it up.
Morale of the story, pop into the docs and get referred to a physio. They should sort you out.
Yep that's it. Literally no pain for 2-3 miles, then it gradually builds up as a dull ache on the outside of my left knee until I can't bear it. I'll pop to the doctors. Cheers for the advice, at least knowing stretching worked for you is encouraging.
I've noticed the outer left heel of my running shoe tends to wear out quicker too, so I'm obviously running with a gammy leg which probably isn't helping.