Where apart from this section the amount of any expenditure on the hiring (otherwise than by means of hire-purchase) of a vehicle to which this Part applies would be allowed to be deducted or taken into account as mentioned in section 375, then the amount of that expenditure shall -
(a) in the case of a vehicle in Category A or B, be increased or reduced, as the case may be, in the proportion which the specified amount bears to the retail price of the vehicle at the time it was made,
(b) in the case of a vehicle in category C where the retail price of the vehicle at the time it was made was -
(i) less than or equal to the specified amount, be reduced by 50 per cent, and
(ii) greater than the specified amount, be reduced in the proportion which 50 per cent of the specified amount bears to that price,
and
(c) in the case of a vehicle in category D, E or F, be nil.