4.The amount of an interim payment under a construction contract shall (subject to paragraph 5) be the difference between—
(a) the aggregate of the gross value (determined in accordance with the construction contract) of the work done under the construction contract at the payment claim date concerned together with any additional amounts in the interim payment under the construction contract, less any deductions from payment provided for by the construction contract, and
(b) the aggregate amount of interim payments that have already been made at that payment claim date.
5. The aggregate of payments made under a construction contract shall not exceed—
(a) the amount provided for in the construction contract as originally concluded, and
(b) amounts provided for by any amendments to that contract agreed between the parties.