Occupational health signed me off today but my boss won't let me return to my original job.
She has given part of it to another member of staff.
I hold a full time permanent post and have been doing the job for 6 years.

Were you out on FMLA?

Normally they just have to return you to work, you are normally not guaranteed your old job back.

If this was a formal leave though and approved FMLA leave, (if your company is covered under it, and if you requested it. And that leave was less than 12 weeks.
How long were you on leave.

But if then, it is only" Employees are also entitled to return to their same or an equivalent job at the end of their FMLA leave"

So if the exact job is not available, it has to be a job equivalent to it, with the same general hours and working conditions.