Having spoken to a couple of ATC bods they seem to believe that if the airfield publishes hours saying 8 till 8 then that is when the ATZ exists, even if they NOTAM a closure. However, CAA docs refer to notified hours and I would have said that if you're notifying the airfield closed by NOTAM then the ATZ goes with it.
As others have said, the ATZ in no invisible force field and self announcing is a valid when entering if no ATC is available.