I haven’t worked for Specsavers for about 4 years now as I cannot do a proper test in 20minutes. It looks to me as though they are trying to get away from using self employed locums and force you into becoming employees. I would write to the Optical press like OT and the Optician. See if they publish your letters. I find this very worrying. Maybe Specsavers can offer you insurance through their own schemes?