So glad you are going to see them Monday SallyG.
Before you do, sit down and write out in a formal way the way your boss is currently allocating your workload versus what you were led to believe were to be the guidelines from Occupation Health.
Writing it out formally (and it might take a few drafts) will take the emotion out of it - which means that you will be calmer on Monday when you go and see Occ Health.
Hand the letter to Occ Health so that you have a written record of your concerns on the record in case things get difficult further down the line (ie cover your ass!).
It might also be helpful to ask whether Occ Health or HR can have a meeting with you and your boss present where they mediate over what your boss's expectations are versus what is realistic and what should be being done.
This might be very scary particularly if there is power dynamic and you feel vulnerable to being let go ... however the flip side is that, if it's possible to have something like this, then its all out in the open and your boss will also be made aware that his behaviour towards you is being tracked.
Have you got a union rep you can lean on for support?
Are other people present when your boss makes unreasonable demands on you with respect to work? If so, would they be willing to confirm this to others (eg Occ Health)?
I really feel for you. I had a very difficult boss in my last work place and I finally lost it with him just before I left (which is most unlike me but I was pushed too far and then some and my co-workers agreed with me). But he had a long history of grievance actions against him and still hadn't and hasn't been removed ... some organisations just don't get it. Unfortunately, I can't now use him for references but his line manager offered to be a referee for me.
Sending hugs xx