Agree about the real cash difference (about 60p/litre saving still). But the petrol price in 2004/5 was between 85-90p/litre depending on where you filled up and, back then, I was getting LPG for around a third the price of petrol which was a no brainer.
If you check the budget treatment of LPG over the past few years, the tax per litre has gone up more on LPG than on petrol by several additional pence and, proportionately, the tax take on LPG has gone up compared with petrol. I contacteded my MP about it, but it looks like the votes in LPG are pretty low priority....
Anyway... it's still worthwhile for many .... and a good way to reduce the cost of motoring