A service provider must not—
(a) enter into an associate contract that has; or
(b) vary an associate contract so that contract, as varied, has; or
(c) give effect to a provision of an associate contract that has,the purpose, or would have or be likely to have the effect, of substantially lessening competition in a market for natural gas services unless—
(d) that associate contract is an approved associate contract; or
(e) that provision is contained in an approved associate contract.