"There is no arguable fairness in [HMRC] pursuing that duty merely because, for some reason, they have failed to pursue their obligations in relation to the other taxpayers. Nor could it possibly be contended that there was unfairness to the other taxpayers since they had the good fortune … to have escaped the tax. But the mere fact that two taxpayers in arguably the same situation have not in fact been charged tax does not raise a case of unfairness without more."