If you’re a Trade Union member, talk to your rep and (via him/her) get to talk to your Union’s expert legal people, so they are on your case. If you’re not a Trade Union member, then hurry up and join (I am assuming here that your employer is local government, so it is a unionised environment.)
As others have said, your employer is legally obliged to make ‘reasonable adjustments’ to get you back to work and help you stay there, despite your MS. It remains the case that people and their employers sometimes have to part company when all reasonable avenues have been exhausted, but that should be a last resort.
Many employers try to apply a one-size-fits-all sickness absence policy to begin with. How much of this is just cynical trying to get away with it, and how much is just good old-fashioned incompetence in the HR department will vary from place to place!
Don’t panic, though. Once the issue is elevated above dim-wit level in the HR department (and getting it elevated through the TU route is a good way of doing this) then you should meet with a more constructive and well-informed response from your employer.