Physio suggests knee pain is biomechanical as little swelling and have altered gait from ankle injuries trying taping and strength exercises?

Agree. If there is no history of trauma and no instability, catching , or locking then this may be mechanical. Therapist don't not diagnose but provide evaluation. A knee specialist can provide you with a formal diagnosis.
Therapy. Knee pain can come from ankle instability. Therapy to strengthen the ankle as well as the knee can help. A brace or orthotic to stabilize your ankle may also be helpful.