“In order to decide whether [the Secretary of State’s supersession decision reducing the rate of both components of PIP to the standard rate] should stand or fall, the tribunal had to determine what it considered the correct rate of both components to be, given the body of evidence now before it. Having concluded that there was no entitlement to either component, the tribunal had no choice but to give a decision to that effect. Anything else (such as leaving undisturbed an award that its findings did not support) would have been futile, incoherent and self-contradictory.”