I'm wondering if maybe you have torn the Sartorius? It seems you have been attacking it quite fervently with the stretching and physio/sports massage, but it isn't letting go completely. Giving you pain 20 mins into a bike ride seems to me too soon for tightness. If anything, I would suspect that the tightness would disappear as your legs got warm and you would present with a painful at the start and the end of the session, but nothing inbetween sort of thing. Have you had any imaging such as an Wikipedia reference-linkMRI done to look into the physical condition of the muscle. Could possibly be a grade 1 or 2 tear and that is giving you issues.