Under an Option E contract you are paid Defined Cost plus Fees. Defined Cost is a term defined at clause 11.2(23) of the ECC which is broken into three elements (1) payments to Subcontractors, (2) cost of components in the Schedule of Cost Components, and (3) Disallowed Cost.
If your client (presumably the Employer and Project Manager?) doesn’t want to pay something as Defined Cost then they would have to justify why by reference to the contract. Is the cost not payable as payments due to Subcontractors? Does the cost not comply with one of the components in the Schedule of Cost Components? Does the cost comply with one of the bullets point under Disallowed Cost?
On your part, you need to demonstrate that the cost you are claiming is actual cost you have paid out usually by provision of an invoice or subcontract payment certificate. You also need to make sure the cost is liable to be paid in accordance with (1) and (2) above and doesn’t fall foul of (3). You also have to demonstrate that the amounts you’ve paid are in accordance with clause 52.1 and are at open market or competitively tendered prices etc etc.
Basically if you have a liability to pay for something as a result of an instruction from the PM, provided you can demonstrate that the cost was incurred and is due for payment under the contract, the Employer / PM doesn’t have much of an argument against you. This said I’ve come across this issue before and you may find getting them to budge on the issue difficult.
Remember in an ECC Option E contract, you the Contractor take very little financial risk, most of the risk is with the Employer and around how they instruct the work they need.