Is it purly LPG or dual? If its purly LPG then you are alternate fuel, however before March 2001 they don't care what fuel it is, so you will be £175, you could have a 14litre scania engine in there and its still going to be £175.
They say the the tax is based on emmisions, they don't actully care about emissions, its just a way to get more of our money, and keep the poor off the roads.