In reply to PPP:
(Not a physio) but I have seen a lot with many running injuries now.
Yes the ITB will not stretch but if it doesn’t stretch then how can it be tight?! This should give you a clue as the ITB is tensioned by various muscles including the glutes and TFL.
Depending on your running history and what you’ve done recently it’s likely you have shocked one of these muscles and it’s now rebelling by being tight. This in turn pulls on the ITB which rubs on the side of the knee (I forget on what it doesn’t matter but this is what causes the pain).
So you likely need to create some slack in the system. If a muscle is tight the you can either stretch or massage. For me both work and I have had varying success with pidgeon stretch (and variations if you are really tight - google) ease into it takes about 2-5 mins to relax into this. To massage get a hockey ball google TFL/glute medius/ glute maximus and then lie/ sit on the ball. This will hurt - do it in one spot until you feel the muscle relax- search for tight spots. If you overdo this it can cause the muscle to tense and cramp for days so warm it up and start slow....
A combination of the above has always solved my issues.
Then you need to go right back to the start and figure why it was tight to prevent it happening again. Too much/ too fast/ too far/ too hilly/ too offf road / too on road. Basically whatever you changed or did you did too much of it.
You can get ahead by doing strength work to prevent and toughen you up google ITBand rehab routine. Or I have found in the last year just hike/ run (depending on level) a load of hills- careful on the downs until you’re robust. This seems to get you a lot stronger than any exercises.