(1) For the purposes of this Act and of every other Act (whether passed before or after this Act) relating to mechanically propelled vehicles, the weight unladen of any vehicle shall, save as is otherwise provided by or under this Act, be taken to be the weight of the vehicle inclusive of the body and all parts which are necessary to or ordinarily used with the vehicle when working on a road, but exclusive of the weight of water, fuel, or accumulators (other than boilers) used for the purpose of propulsion and of loose tools or loose equipment.
(2) In the case of a mechanically propelled vehicle -
(a) of which the weight unladen would exceed seven and one quarter tons when calculated under the foregoing sub-section, and
(b) which is fitted with a permanent or substantially permanent body or superstructure designed and constructed for a special purpose other than the carrying of passengers or goods or any particular class of goods,
the weight unladen of such vehicle shall (save as