NEC SC: What value is used for the application of delay damages?

Under the Supply Contract form the Supplier has failed to achieve the delivery date and are therefore liable to delay damages.

Our clause X7 states that: -
amount per day is £500 limited to 10% of the contract value

As there is no defined term in the contract wording for ‘contract value’ is this deemed to be the contract value at signature or the final contract value taking in to account CE’s?

Other legal arguments may come into play - although I can’t think of any at the moment because it is such a simple sentence - so I think the good old contra proferentum rule comes into play which is (something like) - on a reasonable reading, if there are two or more ambiguous interpretations then it is is interpreted in the light least favourable to the party who created the ambiguity.

In this instance, that would be the Employer. Assuming the Prices have gone up, it would therefore be the original contract Prices at the time the contract was entered into.