4. A modification is a permitted modification if—
(a)the competent authority considers that—
(i)a known risk has materialised otherwise than as a result of any act or omission of the competent authority or the operator,
(ii)because of that fact, the public service contract cannot be performed to the satisfaction of the competent authority,
(iii)the modification goes no further than necessary to remedy that fact, and
(iv)awarding a further public service contract under these Regulations, instead of modifying the contract, would not be in the public interest in the circumstances, and
(b)the modification would not increase the estimated value of the public service contract by more than 50% ignoring, for the purpose of estimating the value of the public service contract, the fact that the risk has materialised.