Annual leave shall become due immediately an employee has completed the qualifying period specified in subclause (2) of this clause but it may be taken before or after it becomes due if—

(a)        the circumstances of the employer's business so require: or

(b)        the employer and the employee so agree:

Provided that annual leave shall in no circumstances be taken more than two months before the due date, nor delayed for more than four months after the due date, unless the employee and the employer concerned have, before the expiration of such period of four months, agreed thereto, in writing, and shall not be delayed by more than six months after the due date.