On the subject of ATZ hours, the ATZ is considered to be active during the published opening hours of the airfield. Outside that the ATZ does not exist unless activated by notam.
The only exceptions will be when the airfield is a "Government Airfield", in which case the ATZ is active H24 regardless of whether ATC are there or not.
The vast majority of flying at Warton is of a of a "Government Nature", and consequently I think that's why they differ.
Don't forget that the MATZ (Military ATZ) surrounding a Government aerodrome ATZ is unregulated airspace, and no formal permission to penetrate it is required - unless it is coincident with controlled airspace - as I think is the case at Warton. That said, it would be very unwise and probably poor airmanship to enter a MATZ without calling to announce yourself first.