If I had a diesel, I think I'd join you and run bio while it's still cheap, but both my cars are petrol driven so I'm stuck. Agree about the conversion costs on LPG, and availability is still an issue.
In theory the point of the stupidly high tax on petrol is for environmental reasons.
That's the fairy tale they spin us to justify the high cost of gas, real reason is that the government knows we don't want to lose the convenience of our cars, so it can use the "environmental good" lie to screw us out of more money. I agree with you that it'll be hard to use that argument with Bio-diesel, but they're an inventive lot, and I'm sure they'll soon find a reason why "for our good" they have to put the price up.