‘[120]…[the appellant’s representative] submitted that [HMRC’s submission] amounts to HMRC being allowed to benefit from their own mistake, since, on21 November 2014 , when they wrote the letter…they had all the information they needed to determine the correct amount of duty. Therefore, the fact that they belatedly realised that they had made a mistake in their calculations should not allow them to benefit from an extended time limit for charging a penalty. [121] The short answer to this submission is that ¶16(4)(b) …does not ask when the amount of duty becomes ascertainable. Nor does ¶16(4)(b) …set a time limit by reference to the date on which specified facts come to HMRC’s knowledge. Rather the relevant question is when that tax became ‘ascertained’.’