Are pay when paid clauses enforceable?

Generally no. A pay when paid clause says you only get paid once the party paying you has itself been paid by someone further up the chain. For contracts covered by the Construction Act such a term is ineffective, with one narrow exception: where the party up the chain who was meant to pay has become insolvent, unable to pay its debts. Later amendments closed a related trick too, so a term tying your Due Date, the date your payment obligation is fixed, to events under a different contract, such as a payment being certified higher up the chain, is also ineffective in most cases. Where knocking out such a clause leaves no proper way of working out payment, the fallback Scheme for Construction Contracts fills the gap. So being told to wait because the employer up the line has not paid is usually the other side managing its own cash flow, not something the contract entitles it to do.

An electrical subcontractor chases £30,000 and the main contractor points to a clause saying it need not pay until the employer has paid it. On a contract covered by the Act that clause is generally ineffective, so the subcontractor's right to be paid stands on its own timetable whatever the employer has done. The one situation where the contractor could rely on it is if the employer above had become insolvent. A cousin, a clause making the subcontractor's Due Date wait until the employer certifies payment up the chain, is usually struck out too, and where removing it leaves no workable payment mechanism the Scheme's default dates step in. So being told to wait because the money has not come down the line is usually the contractor managing its own cash flow, not a contractual right the subcontractor must accept.