If time is ‘at large’ at the commencement of an NEC2 contract (because both parties accepted N/A Contract Completion Dates), can a contract Completion Date be imposed subsequently based on email correspondence detailing a proposed installation date and expected leadtime?
It depends what was in the emails !
If you have made statements which explicitly agree or accept a proposed Completion Date, then ‘Yes’.
However, agreeing to work to “installation dates” and “lead times” sounds more like operating the contract to make reasonable progress, so the answer is probably ‘No’.
It would also require a change to the contract under clause 12.3 for it to be effective.
Clause 12.3, states that, " no change to this contract, unless provided for by the conditions of contract, has effect unless it has been agreed, confirmed in writing and signed by the Parties".