A new emissions based motor tax system was introduced in 2008, together with changes to Vehicle Registration Tax, in order to influence the future purchasing decisions of car buyers. The owners of vehicles, including dual fuel vehicles, registered prior to 1 July 2008 continue to pay motor tax on the basis of engine capacity. Tax on dual fuel vehicles, which do not have zero levels of carbon emissions, is based on the CO2 emissions level of the vehicle, as shown on the manufacturer's Certificate of Conformity, if registered after that date.
I do not anticipate that the current review of the CO2 banding and rates for motor tax for post-2008 vehicles will change the basis for charging motor tax for dual fuel vehicles.