NEC ECC: Changing contract start date

We were awarded a contract for works in March, we started getting design works completed and visited site to complete a few activities in May after an initial delay by the Employer.
We then were off site until September. This was not our fault.
We were then issued a communication that stated September would now be our start date for the contract duration.

My question is, if we were actually awarded the contract (in writing) in March and were on site in May can they then change the contract start date? If so, in what form would this communication have to take place?

The simple answer is no.

From your question it is not clear if you have not been provided access or if effectively you have been instructed to stop or not start any work. The former would be a CE subject to the provisions of clause 60.1(2), the latter a CE under clause 60.1 (4).

It appears that you should call a risk reduction/early warning meeting to try and get clarity on whats happening.

I would be less worried about when the starting date was and more worried about what the Completion Date should be. Starting date is in effect contract award so that would be the March date even though you haven’t done much. Therefore it is the Completion Date that needs moving, and for you to justify the cost associated with it moving out within that compensation event (Assuming they agree it is one).

As soon as you have been delayed access and it is not your fault that should be notified as a compensation event. You can then assess the impact against planned Completion which will then allow Completion Date to be moved (assuming the PM agrees in the assessment).