15. A determination of the Secretary of State (including a determination made following a change of circumstances) whether a person is, or is to be treated as, capable or incapable of work shall be conclusive for the purposes of any decision which falls to be made—E+W+S
(a)under a relevant enactment; or
(b)on a claim for or award of—
(i)housing benefit; or
(ii)council tax benefit.