How do I check a payment certificate is right?

Check the document before the arithmetic. A Payment Notice or certificate has jobs to do under the contract and the Act: it must arrive inside its window, state the sum the payer considers due, and show the basis on which that sum is calculated. A certificate that is late, or offers a bare figure with no basis, may not be a valid notice at all, and that changes what you are owed regardless of its content, so establish validity first, with the contract open. Then reconcile it against your application line by line, not total against total, because one overall number can hide offsetting movements, a fair assessment of measured work masking an unexplained deletion of a variation. Sort the differences into three piles: cuts you accept, cuts the records can answer, and cuts with no stated reason, which deserve a pointed question, since the obligation to show a basis exists precisely so sums cannot be shaved namelessly. Finally, answer inside the cycle. A short written response, line, amount, why the cut is wrong, sent while that certificate's cycle is open, keeps the difference alive; a year of unanswered certificates reads, at the final account, like a year of agreements.

Take a roofing subcontractor whose certificate arrives four days after the contractual Payment Notice date, cutting the application hard: the measured work trimmed with reasons, one variation halved with none, and the whole thing arriving with a compliments slip rather than any stated basis. Checked casually, it is a disappointing number to grumble about. Checked properly, it is three findings: a notice that is late and arguably invalid, altering what is due for the whole cycle; a variation cut with no basis stated that cannot survive a written challenge; and a measurement difference on the tiling that the marked-up drawings can settle in an afternoon. The subcontractor's two-page reply, sent within the week, records all three. The next certificate arrives on time, states its basis, restores most of the variation, and the measurement difference resolves at a site walk. None of it required a dispute; it required the certificate to be read the day it landed by someone who knew what it had to contain.