I've been given a contract 6 months into a job role that I signed and handed back a few weeks ago. HR have since come back and said that the pay should actually be lower & want to give me a new contract. Can they do this?
You would have to accept the terms, but the reality is that if you don't they can get rid of you without much (any) comeback within the first 2 years of employment.
I think you've got a much better case having been promoted and already paid at the new rate for 6 months. I don't think they have a leg to stand on really.