Few points of note here:
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Early Warnings are for things that could or might occur, which then allows the chance to review them and do something about it. This was not an early warning, but should have been an instruction to say you can not access site tomorrow until a different time. That instruction is a new constraint, and if it caused cost and/or time effects to the Contractor that could not be mitigated would be recoverable.
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verbal communications carry no weight under NEC contracts
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much better to be using a cloud based administration tool so that it is much clearer to everyone on the project what has been communicated and when. Emails do comply with clause 13.1 in as much as they are in a form that can be read, copied and recorded but are by no means the best form of communication,
I think take this as an opportunity for both Parties to understand how they can communicate better and make sure such matters do not occur again that could be much bigger issue. I think the Contractor would have a reasonable claim here as the chances are they might not have been able to do much about it anyway with less than 24 hours notice. Practically speaking they should have followed up the email for such an important point and to make sure it is understood.