Where a person is charged with having contravened paragraph 2(1) above, in relation to a large shop which was permitted to be open for the serving of retail customers on the Sunday in question, by reason of his having served a retail customer after the end of the period during which the shop is permitted to be open by virtue of paragraph 2(3) above, it shall be a defence to prove that the customer was in the shop before the end of that period and left not later than half an hour after the end of that period.