11. (1) On receiving any return made under section 10, the Collector shall assess the amount of the duty of excise payable under section 3 in respect of the period to which the return relates, and if the amount has not already been paid, shall cause a notice to be served upon the owner of the mill requiring him to make payment of the amount assessed within thirty days of the service of the notice. (2) If the owner of any mill fails to furnish in due time the return referred to in sub section (1) of section 10 or furnishes a return which the Collector has reason to believe is incorrect, or defective, the Collector shall assess the amount, if any, payable by him in such manner as may be prescribed, and the provisions of sub section (1) shall thereupon apply as if such assessment had been made on the basis of a return furnished by the owner: Provided that, in the case of a return which he has reason to believe is incorrect or defective, the Collector shall not assess the duty of excise at an amount higher than that at which it is assessable on the basis of the return without giving to the owner a reasonable opportunity of proving the correctness and completeness of the return. (3) A notice under sub section (1) may be served on the owner of a mill either by post or by delivering it or tendering it to the owner or his agent at the mill.