“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) as being due to him as a VAT credit an amount which ought not to have been paid or credited, or 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.”