5.—(1) Where an operator is authorised by an environmental permit to operate an installation at which Part A(1) activities, Part A(2) activities or Part B activities which are described in Part 2 of this Schedule by reference to a threshold (whether in terms of capacity or otherwise) are carried on, the installation does not cease to be a Part A(1) installation, a Part A(2) installation or a Part B installation, as the case may be, by virtue of the installation being operated below the relevant threshold unless the permit ceases to have effect in accordance with these Regulations.