" Where a person has failed …….to keep any documents and afford the facilities necessary to verify such returns or where it appears to the Commissioners that such returns are ….. incorrect, they may assess the amount of VAT due from him to the best of their judgment and notify him."
" In any case where, for any prescribed accounting period, there has been paid or credited to any person- (a) as being a repayment or refund of VAT; or (b) …. an amount ……which would not have been so paid or credited had the facts been known or been as they later turn out to be, the Commissioners may assess that amount as being VAT due from him for that period and notify it to him accordingly."