How do JCT Payment Notices and Pay Less Notices work?

In SBC/Q 2016 the Interim Certificate is the Payment Notice, the Contractor's Interim Payment Notice stands in when no certificate is issued, and the Pay Less Notice is the Employer's only route to paying less than the sum those notices state. The pattern is the Construction Act's: section 110A requires a payment notice stating the sum considered due and the basis of its calculation, section 110B gives the payee a notice in default where the payer's notice is not given, and section 111 requires the notified sum to be paid by the final date for payment unless a pay less notice is given in time. Under the interim payment provisions in section 4 the Architect/Contract Administrator's Interim Certificate, issued not later than five days after the due date, is the payment notice. If no Interim Certificate is issued in that period, the Contractor's own interim payment application, where it made one, takes effect as its Interim Payment Notice, and where it made none the Contractor may give an Interim Payment Notice at any time after the period has passed, in which case the final date for payment is pushed back to reflect how long after the period the notice was given. The Pay Less Notice is given by the Employer, or by someone the Employer has authorised to give it on its behalf, not later than five days before the final date for payment, and must state the sum the Employer considers due at the date of the notice and the basis on which it was calculated. What goes wrong is nearly always one of three things: an Interim Certificate issued late and treated as if it still controlled the sum, a Pay Less Notice served before the final date but outside its own window, and a certificate that gives a figure with no basis, which invites a challenge to the notice rather than to the valuation.