Cheers Sam, but i fear that this is where the "loophole" comes in as this is not classed as a temporary structure, but as a "caravan" therefore under the 1968 caravan site act the dimensions it cannot exceed 3.05m from floor to ceiling (at it's highest point) internally, there is no mention of any external height in the act. This is probably because mobile homes used to be just a square box.
And DH thanks also, but it is a weird one as this is not "really" a planning application, but a "certificate of lawfulness" application, which I am lead to believe is different.