QBD 7 July 1988. A dispute between a windows sub- subcontractor and the subcontractor under NAM/SC which occurred when the defendants (D) had not met in full the plaintiffs (P) applications for monthly interim payments . Under clause 19 the amount was to be `the total value of work properly executed by the subcontractor` subject to a retention`, here 3%. D were entitled to a 2.5% discount if they paid on time and payments were due "during the third week following the last Saturday in each month` with leeway of 17 days. The last payment was due on 18 July 1987 but by a letter D for the first time challenged P`s rights to interim payments and invoked a claim to set-off because of delays to the main contract caused by P. The case was held in favour of P on the grounds that under clause 21.4 set off was to be governed exclusively by the contract and not by subsequent letters. He gave leave for D to counterclaim.