“As a general principle, the IC was, in the Tribunal’s view, entitled to accept the word of the public authority and not to investigate further in circumstances, where there was no evidence as to an inadequate search, any reluctance to carry out a proper search or as to a motive to withhold information actually in its possession. Were this to be otherwise the IC, with its limited resources and its national remit, would be required to carry out a full scale investigation, possibly onsite, in every case in which a public authority is simply not believed by a requester.”