What happens if the payer misses the Payment Notice deadline?

The payee's paperwork takes over. A Payment Notice, the payer's statement of the sum due for the round, has 5 days from the Due Date, the date the payment obligation is fixed. Miss that deadline, and where the contract lets the payee apply and it applied properly, that application stands as the notice by default, its figure becomes the Notified Sum, the amount that legally must be paid, and the Final Date For Payment, the last day the money can lawfully arrive, does not move. Where there was no such application, the payee can serve its own default notice after the window closes, and the Final Date For Payment is pushed back by however many days that notice was late. Either way the payer has one card left, a valid Pay Less Notice cutting the sum down, served before the prescribed period ahead of the Final Date For Payment; miss that too and the applied sum is payable in full. That last situation is the smash and grab, so serve each round's notices on time.

A steelwork subcontractor applies for £150,000 on the Due Date, a Wednesday, on a job where the contract expressly allows applications. The short staffed main contractor lets the 5 day window pass without a word. Because the application was proper and on time, it stands as the notice by default: £150,000 becomes the Notified Sum and the Final Date For Payment does not move. The contractor still has one route, a valid Pay Less Notice served before the 7 day mark ahead of that Final Date For Payment, cutting the figure to what it can justify. Miss that too and the whole £150,000 is payable and enforceable in a smash and grab adjudication.