7. If the total amount of tax which a taxable person who is under a duty to make payments on account was liable to pay in respect of the prescribed accounting periods the ends of which fell within any one period of one year ending after the end of the basic period was less than £1,600,000, then, with effect from the date of the written approval by the Commissioners of a written application by the taxable person to that effect, he shall not be under a duty to make payments on account.