I have been employed as a painter and decorator for 8 months on a sub-contract basis. Recently my boss found out that I don't have official Trade Papers. Now he wants to cut my hourly rate because I am not a 'Qualified painter'. I have been painting for 40 years on and off. I decided to not take a pay cut and no longer work for him. He now refuses to pay me for my last invoice at the previous rate consisting of 2 weeks completed work. Is this legal?