if the engineer fails to certify in accordance with sub-clause 14.6 [issue of interim payment certificates] or the employer fails to comply with sub-clause 2.4[empolyer's financial arrangements] or sub-clause 14.7[payment], the contractor may, after giving not less than 21 days' notice to the employer, suspend work [or reduce the rate of work] unless and until the contractor has received the payment certificate, reasonable evidence or payment, as the case may be and as described in the notice.