In section 4 of the 1971 Act (exemptions from duty) in subsection (2) for the definition of “tower wagon” there shall be substituted the following—

““tower wagon” means a goods vehicle—

(a)

into which there is built, as part of the vehicle, any expanding or extensible contrivance designed for facilitating the erection, inspection, repair or maintenance of overhead structures or equipment, and

(b)

which is neither constructed nor adapted for use nor used for the conveyance of any load other than—

(i)

such a contrivance and articles used in connection therewith, and

(ii)

articles used in connection with the installation or maintenance, by means of such a contrivance, of materials or apparatus for lighting streets, roads or public places”.