Are you left handed? Just wondering about the pain distribution down the arm on the left side?
Has the physio looked at muscles like the scalenes that go from the transverse proccesses of the neck vertebrae to first and scond rib?
They can have trigger points in them that give your pain distribution symptoms.
Indications: back,shoulder and arm pain. Thoracic outlet sydrome. Scalene syndrom. Oedema in the hand. Whiplash. 'restless neck'. Irritability.
Referred pain patterns:
anterior: persistant aching, pectoralis region to the nipple
posterior: upper medial border of scapula
lateral: front and back of arm to the thumb and index finger
Differential Diagnosis: Brachial Plexus. Subclavian vessels. Cervical discs c5 c6. Thoracic outlet syndrom. Angina. Carpal tunnel. Upper trapezius. Sternocleidomastoid. Splenius capitis.
They can be activated by various things. Often pulling or lifting. Relief sometimes comes if you hunch shoulders towards ear or let underarm be supported on something under your ellbow when sitting to reduce the pulling downwards effect when they are aggravated. Having had a whiplash all sorts of factors can be involved. Driving could have stirred them up. Moist warmth but not hot hot hot around neck may help.
Gentle stretch of them like move ear /shoulder away from each other (on the same side ) can help as part of a treatment but diagnosis online is an iffy affair and I can't be sure if it is that or what your physio is doing with you.