Hi There,
It would depend on why the stiffness is there.. is it due to soft tissue contracture due to immobilisation or disuse, or is it swelling/oedematous, or other structural impediments. Have you queried whether surgery would consider a manipulation under anaesthetic if you're really stuck as it has been a long time since cast was removed. If it's just due to short tissues and stiffness that way i would be very aggressive with prolonged stretching and kneeling back onto heels as far as possible to sit for as long as possible (ie. 15 minutes at a time) to cause actual tissue fibre lengthening (rather than a short term stretch response which occurs with short duration stretching and mobilisation). also check that the rotational component of tibia on femur is occurring correctly to enable full flexion and full extension of the knee.
Hope that helps.
msk101