HMRC must make any assessment to the best of their judgment (Value Added Tax Act 1994, s. 73(1)). In the leading case on this point, Van Boeckel v C & E Commrs (1980) 1 BVC 378, it was held (at p. 380) that this means:-
‘the commissioners will fairly consider all material placed before them and, on that material, come to a decision which is one which is reasonable and not arbitrary as to the amount of tax which is due. As long as there is some material on which the commissioners can reasonably act then they are not required to carry out investigations which may or may not result in further material being placed before them.’
In Spillane (1988) 3 BVC 1,417, it was held that the tribunal was justified in finding that the VAT officer who made the assessment, when faced with difficulties in getting information about the extent of the appellant’s business, was entitled, when making the assessment to the best of his judgement, to use figures towards the top of a possible range, leaving the appellant to show that this was incorrect.
It is not enough to show that the officer responsible for the assessment was negligent to establish that Customs failed to exercise best judgement. In Pegasus Birds Ltd v C & E Commrs  BVC 68, the court held that there had to be a conscious or irrational failure to place a reasonable interpretation on the evidence to hand.