S. 15A inserted (10.7.2003 with effect as mentioned in s. 16(3) of the amending Act) by Finance Act 2003 (c. 14), s. 16(1)
Where—
a vehicle licence has been taken out for a tractive unit, and
the licence was taken out at a rate of vehicle excise duty applicable to a tractive unit which is to be used with semi-trailers with a minimum number of axles,
duty at a higher rate does not become chargeable under section 15 by reason only that while the licence is in force the tractive unit is used with a semi-trailer with fewer axles than that minimum number, if the condition in subsection (2) is satisfied.
The condition is that the rate of duty at which the licence was taken out is equal to or exceeds the rate which would have been applicable if the revenue weight of the tractive unit had been a weight equal to the actual laden weight, at the time of the use, of the articulated vehicle consisting of the tractive unit and the semi-trailer.