An older friend of mine is a cancer patient, who is finally on the road to recovery (for now).

Despite not being able to go out due to her condition, which led to other problems, she was knocked back on multiple occasions for DLA (or it may have been ESA - something along those lines), and consequently knocked back for a Concessionary Bus Pass too. You hear similar stories in the news all the time, of folk having their benefits revoked and these individuals being made to find work.

Obviously PIP will be different to the benefit above, but I'm guessing that you'll still have difficulty with the claim.