(1) If, after the date on which an agreement referred to in section 13C(1)(b)(i) is signed by the employer and the employee (whether or not the approved flexible working arrangement to which it relates has commenced), the employer and the employee so agree, in writing -

(a) the flexible working arrangement or part of it may be postponed to such time as may be agreed to,

(b) the period of the flexible working arrangement may be curtailed in such manner and to such extent as may be agreed to, or

(c) the form of the flexible working arrangement may be varied in such manner as may be agreed to,

and in such a case the agreement referred to in section 13C(1)(b)(i) shall be deemed to be amended accordingly.