When St Mawgan becomes a civillian airfield the MATZ will go immediately and it will also cease to be a Government Aerodrome.
The ATZ will be active in accordance with the published hours of aerodrome operation, i.e if the airfield is published open H24, then so will be the ATZ.
That said, with the staffing of ATC which was originally proposed by the management, there's not a hope in hell of operating a full ATC service H24.