I think you do but I think it depends on what car you have converted.
I looked into this and there is a list of cars which when converted are known the produce less CO2 and fall into the excempt catagory. They are only the really naff cars though with tiny engines.
I've done my sums now and don't think it is worthwhile for myself. It would be great as a long term thing but it takes so long to get your money back you won't really be that much better off.
What it does do though is make you think you are saving money because whenever you fill up it will be costing you neary half.
One thing you could consider is buying a car which is already converted. I looked yesterday and there are loads of big saloons going for between £1000 and £1500. They don't seem to be worth anymore than a normal car because people are scared of LPG so they are good value second hand buy's