How do I dispute a contra charge?

Take it in three passes. First, process: a contra charge, a backcharge for something like damage, attendance or supervision the payer says you caused, is still a deduction, so if it was knocked off a Notified Sum without a valid Pay Less Notice served in time, it fails on the machinery alone and the shortfall is recoverable whatever its merits. Second, entitlement: find the clause relied on, check any conditions it carries, notice of the cost before it was incurred being a common one, and ask whether the charge is a genuine loss or a tariff; charges invented at final account for events nobody documented at the time have a short life under scrutiny. Third, quantum: demand the substantiation, the invoices, timesheets and allocation behind the number, because contra charges are routinely round numbers that dissolve when itemised, and check for double counting against work you were never paid for. Put the challenge in writing while the events are fresh, and if the charge stands unpaid after that, adjudication decides it quickly; assessors know that, and a documented challenge often ends with the charge quietly dropped at settlement.

Say a painting subcontractor finds £18,000 of contra charges in the final assessment: scaffold adaptions, operative supervision, and making good damage, all in round thousands, none mentioned during the job. Its challenge letter does three things: notes that none of the charges ever came through a Pay Less Notice in any cycle, asks for the clause relied on for each, and demands the invoices and timesheets behind the numbers. The substantiation that comes back covers £6,000, the scaffold company's actual invoices; the supervision charge turns out to be an internal allocation with no records, and the damage was signed off as made good at handover. The account settles with £5,500 of charges standing. The lesson is that contra charges are priced for the subcontractor who never asks.