Complex sentence: Since then a'New Generation' of road vehicles based around the VE Commodore have been developed, called the "Heritage Series"... these'Retro' cars are based upon iconic HDT special vehicle designs of the Brock era.
Split into simpler sentences: Since then a'New Generation' of road vehicles based around the VE Commodore have been developed. HDT's VE Commodore sedans form the new "Heritage Series" and these "Retro" cars are themselves based upon iconic HDT special vehicle designs of the Brock era.