The only thing that surprises me about this is that anyone is surprised that it's happening.
As an aside, while there may have been one or two people who bought or converted cars to run on LPG because of the lower CO2 emissions, I would postulate that the vast majority did so to benefit from the lower taxation - be that on the fuel, the VED or the London C-Charge.
Once again it has been demonstrated that taxes do nothing but distort the market and raise revenue.