How to send a late Payment Notice

You cannot. The 5 day window for a Payment Notice, the payer's statement of the sum it thinks is due, runs from the Due Date, the date the payment obligation is fixed, and is set by law, so a notice served on day 6 is not a Payment Notice at all. Once it lapses, the payee's application, or a default notice it can serve when the payer stays silent, is in position to become the Notified Sum, the amount that legally must be paid. Your remaining move is a Pay Less Notice, cutting the sum down, served up to the prescribed period before the Final Date For Payment, the last day the money can lawfully arrive; where the contract is silent, the Scheme for Construction Contracts sets that period at 7 days. Miss that too and the full Notified Sum is payable, enforceable through adjudication, the fast 28 day dispute process, so the fix is to serve the next round's notices on time.

A developer's site team only realises a week after the Due Date that no Payment Notice went out against a fit out contractor's £90,000 application. Backdating one achieves nothing: served late, it is not a Payment Notice, so the £90,000 is lined up to become the Notified Sum. The team can still serve a Pay Less Notice, provided it lands at least 7 days before the Final Date For Payment, stating say £71,000 with the deductions worked through. If even that has gone, the full £90,000 is payable and could be chased in adjudication.