Do you stretch after riding and after you've warmed up? There's loads of quad stretches you can do eg:
This is the normal one:
A variation on this is where you do that stretch with a chair or bed behind you, rest the foot of the bent leg which is behind you on it, and then bend the straight leg. It's a killer, really good for quads and psoas.
This one atrociously painful but really good. It's called the 'bow pose':
I wouldn't try this at home from cold. I always do it as part of a pilates class with an instructor there.
But if you want some really good ones and a regime of stretches you can follow, the physio will more than likely make recommendations. If not you should ask them. I have had some excellent sports physio in the past which featured some fairly strict and intense stretching regimes alongside other treatment and it worked for me.