“… in assessing the meaning, weight and effect reasonably to be given to statements of the revenue the factual context, including the position of the revenue itself, is all-important. Every ordinarily sophisticated taxpayer knows that the revenue is a tax-collecting agency, not a tax-imposing authority. The taxpayer's only legitimate expectation is, prima facie, that he will be taxed according to statute, not concession or a wrong view of the law: Reg. v. Attorney-General, Ex parte Imperial Chemical Industries Plc . (1986) 60 T.C.1, 64g, per Lord Oliver of Aylmerton. Such taxpayers would appreciate, if they could not so pithily express, the truth of the aphorism of “One should be taxed by law, and not be untaxed by concession:”