5. The final payment payable under a relevant construction contract, namely the payment of an amount equal to the difference (if any) between—
(a)the contract price, and
(b)the aggregate of any instalment or stage or periodic payments which have become due under the contract,
shall become due on the expiry of—
(a)30 days following completion of the work, or
(b)the making of a claim by the payee,
whichever is the later.