17(1)Where an amount is due from any person on account of the tax, but by reason of his failure to keep or to produce or furnish any records, accounts or other documents as required by or under this Schedule, or of his failure to take or permit to be taken any other step so required, or by reason of such records, accounts or other documents being materially incomplete or inaccurate, the Commissioners are unable to ascertain the exact amount of tax due from him, the Commissioners may assess the amount due from him to the best of their judgment and notify it to him.
(2)An assessment under this paragraph of an amount of tax due shall not be made after the later of the following:—
(a)two years after the time when the amount became payable; or
(b)one year after evidence of facts, sufficient in the opinion of the Commissioners to justify the making of the assessment, comes to their knowledge;
but may, where further such evidence comes to their knowledge after the making of such an assessment, be made in addition to that assessment; but no such assessment shall be made more than six years after the time when the tax became payable, except for the purpose of recovering tax lost to the Crown through the fraud or wilful default or neglect of any person.
(3)An amount assessed and notified under this paragraph shall be recoverable as an amount of tax due unless in any action relating thereto the person liable proves the amount actually due and that amount is less than the amount assessed.