I understand how difficult this must be for you as you work in an overstretched department. However, the fact that your section is understaffed is a management issue and it is incumbent upon them to rectify the matter, rather than put your health at risk by asking you to work outside the recommendation of the OH report.
Your union will have a legal team experienced in matters of employment law. Have you asked your local representative to escalate the matter on your behalf with its own lawyers? Not only is your MS taking its toll on you physically but the accompanying mental stress you are experiencing is also exacerbating your symptoms.
Whilst this issue is being resolved have you not considered seeing your GP and obtaining a sick note? The new sick note or ‘fit note’ can specify the circumstances under which you can work. You can discuss these circumstances with your GP and they can be listed on the note. You could take some time off to recuperate and then return on your terms. In the meantime your union should be fighting to help you cement these changes into a revised work pattern.
You may already have tried all the above, of course.