Taking away my points.

Hi phrank,
The law provides that if DWP receive a new medical report that is one if the reasons that can permit a change to a lesser benefit award, including no award at all. So what happens, is that DWP receive a poorly administered assessment and use this to justify zero points.
However, the law also says that a difference of medical opinion isn't the same as a change if circumstance ie an improvement. DWP should still carefully analyse the descriptors and see which really do apply. Of course DWP usually fail at this point.
The role of the tribunal is to make the correct that DWP should have lawfully made. So all powers etc that DWP has are shared by the Tribunal.
The tribunal will need to ensure that the supersession is legally made, and if it feels that the lesser award is correct it has to make a very cogent explanation as to why, and that would probably mean also explaining why it agrees with the assessment report rather than eg GP or consultant report from the claimant's doctors. Needless to say if the tribunal don't apply the correct law then there is an appeal to the Upper Tribunal!
There is also a power to supersede if DWP think a change if circumstances has actually occurred. This is more complicated. DWP have to prove the change. This might be in a fraud case where eg surveillance shows improved walking. It might also be where DWP have had a report from the GP which seems to contradict the claimant. In the change if circumstances scenario, DWP also have to prove the effective date, ie when the change took place, this can be a tricky thing to prove. That's why its easier to use the new medical report law!
I hope this helps.
LL26