11Where an amount is due on account of general betting duty from any person, but by reason of his failure to keep or to produce or furnish to the proper officer the accounts, records or other documents required under or by virtue of this Schedule, or to take or permit to be taken any other step which he is so required to take or permit to be taken, or by reason of the accounts, records, or other documents kept, produced or furnished being materially incomplete or inaccurate, the Commissioners are unable to ascertain the amount of duty properly due from him, the Commissioners may estimate the amount due; and (without prejudice to the recovery of the full amount due or to the making of a further estimate in that behalf) the amount estimated shall be recoverable as duty properly due unless in any action relating thereto the person liable proves the amount properly due, and that amount is less than the amount estimated.