The aforementioned GDP Order 2015 states:-
Permitted development
A. The provision on land of buildings, moveable structures, works, plant or machinery required temporarily in connection with and for the duration of operations being or to be carried out on, in, under or over that land or on land adjoining that land.
Development not permitted
A.1 Development is not permitted by Class A if—
(a)the operations referred to are mining operations, or
(b)planning permission is required for those operations but is not granted or deemed to be granted.
Conditions
A.2 Development is permitted by Class A subject to the conditions that, when the operations have been carried out—
(a)any building, structure, works, plant or machinery permitted by Class A is removed, and
(b)any adjoining land on which development permitted by Class A has been carried out is, as soon as reasonably practicable, reinstated to its condition before that development was carried out.
I can't find any definition of what is meant by the term "temporary" - it could go on for years, provided the structure is temporary!
The 28 day rule appears to relate to motorsport events or outdoor markets.