Hay Blue
Its is rare that I write a short post but here we go. (P.s. I am going to assume your physio has had another look, assessed your leg and it is his opinion it is scar tissue?)

If you have local scarring in a muscle bulk there are a few things you can do to improve the situation.

1. Stretch (you are doing this already )
2. Exercise the muscle into increased muscle ranges (inner, middle and outer range). So... a simple example might be squats. Start doing static squats against a wall (ball behind your back). Start with small squats (so your knees are @ about 45 degrees at their lowest point), you want to do these controlled etc. but fairly quickly to keep them semi-functional for return to sport. (Don't do a hold @ the bottom, unless you get discomfort from the quick change in direction) Slowly progress this so you are going deeper and deeper.

Once you are @ approx 90-100 degrees at your knees, doing 30 squats with nil symptoms, 3 sets, then move to static squats away from the wall, then add some weight (dbells/bar) then start on single leg squats no weight , etc etc. every time you add a component, start @ 45 degrees again, and work the exercise down to 90 degrees symptom free over time.

3. Some more functional exercises may be: butt kicks, high knees, etc. starting @ a low intensity and working up.

Basically, the more you RANGE your muscle belly during functional activities and exercise the more it will help to reduce your local scarring. (if you don't understand this concept let me know I will explain it out more, just wanting to keep this semi-short, already doing a horrible job @ that)

I am not sure exactly where you are currently @ in terms of Symptoms etc so I can't give you a program to try off the bat. If you give me some more information of some current exercises etc and how you tolerate them, I might be able to

Always warm the muscle up if you can and start low key and build up during your ex. programs when you have local scarring to decrease reinjury.

Sorry, no journal articles etc. all off the top of my head there, if you want some light reading let me know I am sure I can find some.

Thanks!

Musculoman