Here we go.
http://www.direct.gov.uk/prod_consum_dg/groups/dg_digitalassets/@dg/@en/@motor/documents/digitalasset/dg_183952.pdf
CLASS TYPE OF VEHICLE AND/OR USE
Private/Light Goods (PLG) Vehicles registered before the 1.3.01: Private motor cars; goods vehicles not more than 3,500kg revenue weight; vehicles used for “private” (non-trade or business) purposes (including 3 wheeled vehicles over 450kg unladen). Vehicles registered after 1.3.01: vehicles that do not come into the scope of the first six tax classes, listed below.
Petrol Car Effective from 1.3.01. Passenger vehicle in the M1* category weighing not more than 3,500kg revenue weight. Tax rates based on individual vehicle’s CO2 value. Propulsion must be petrol.
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*M1 – as defined in EC Directive 70/156 “vehicles used for the carriage of passengers and comprising no more than eight seats in addition to the driver’s seat”. CO2 emission is ascertained as part of the Type Approval process, and is recorded on individual vehicle’s Certificate of Conformity.
So for a 2008 motor, “Petrol Car” is what you want. Seems PLG is still around, just that it’s now a catch-all for vehicles that fall outside of the more specific definitions.