HMRC’s powers to make assessments to recover VAT are in Value Added Tax Act 1994, s. 73.
HMRC may make an assessment to the best of their judgment.
If new evidence comes to light which indicates that the assessment is inadequate, HMRC may make an additional assessment (VATA 1994, s. 73(6)).
Even where no new information is available, HMRC may, otherwise than in circumstances falling within VATA 1994, s. 73(6)(b) or 75(2)(b), increase the amount of the original assessment if it appears to them that the amount which ought to have been assessed exceeds the amount assessed: a supplementary assessment is made in the amount of the excess on or before the last day on which the original assessment could have been made.
Any assessment must state the reason why it has been made.