PFI and outsourcing disputes are becoming more common. On such projects, some employers keep a keen eye on performance monitoring, the payment mechanism and its deduction regime. Some adopt a less rigid approach. Others wax and wane between the two, depending on who is in charge of the purse strings. The ‘waxers and waners’ should take note of the decision in Compass Group v Mid Essex Hospital Services NHS Trust. In that case the judge said that the Trust should have exercised its discretionary powers under the payment mechanism consistently. He decided that the Trust had not done so. This amounted to a material breach of the contract, particularly the express contractual duty to co-operate with Compass. The case, therefore, reminds us that if the contract requires the parties to co-operate, taking a consistent approach should help to avoid a costly legal tussle.