In reply to Somerset swede basher:
I've struggled a lot over the years off and on, fine on long flat runs, or short hilly ones but long mountain ones finish it off again.
Bit the bullet recently and went to see sports physio. For me the problem stemmed from feet turning out (duck) which I had worked hard to correct but hadn't paid attention to the knee. So for me my right knee was falling in causing pull along the itb as it tried to support my thigh. This is caused by weak gluteal muscles, the small and lateral ones, you can check this yourself - stand before a mirror and do a 1 leg squat keeping your heel on the ground, your knee should be over your foot, whereas mine bent inwards loads.
It's all well and good being told to do squats lunges etc but its having good form that makes it worth it.
One legged squats, lying down straight side leg raises, single leg bridge, lunges and when running pay attention to foot and knee direction.
Decrease stride length and do shorter faster steps puts less stress on the muscles, don't over do it with stretching it can do more harm than good.
Hope this helps
Edit - swimming would help as it takes all pressure off the joint and works the muscles, FC would help glute major and lower back which us important, breast stroke would work more glute maj and minor, as would sidestroke, go for it!
Edit, edit - leave those swedes alone!
Post edited at 22:59