I hope you don't mind me chipping in. In reference to the OPs specific problem, a very common one I'd imagine:
The exemption covers people who would be likely to endanger themselves or others in the event of being found fit for work or fit for work-related activities, right? So people at risk of suicide or violence towards others shouldn't be being called. So if someone is then wrongly assessed as fit to work and one of these consequences results, their death and/or someone elses death or serious injury, assuming that ATOS had had fair warning beforehand that this was likely, would ATOS or the DWP be held responsible, legally?
I know that they would be responsible morally, I'm just wondering if the law reflects morality in this case, at least in theory.