Hello Pammy and welcome to the forum.  I am not clear whether your job is not sustainable - not clear of the details as to what this actually means in practice -  because of changes affecting your company or whether you feel that it is because of issues relating to your illness and its effect on you.

The law states that you should not be discriminated against because of your illness and I attach some information that spells out what this means.  Perhaps you could tell us a few more details, for instance about what is happening at your work, what changes are being made and how the rest of the workforce are affected?  Do you have a trades union at work?

Sorry you are having to go through this problem now after coming through your cancer. 

www.cancerresearchuk.org/.../the-disability-discrimination-act-and-cancer

Annie