What ^ he said.
Follow the chain of command - Been seen to follow procedure. If you start feeling resistance, then take it to HR. I'd probably take it to them anyway. But always remember, HR is there to protect the employer, not the employee. However, in this case, they're on thin ice as they're ignoring medical advice and not making reasonable adjustments. Well, they did, but are now revoking them!
Were you referred to an occupational health organisation? When I was off for a few weeks with a bad back, I had a visit from occupational health to see what, if any, adjustments needed to be made to my work environment to improve my situation. They're involved to protect the employer as much as help the employee.
Are you in a union? They would be able to advise on a course of action.