It does not matter much, the government will make sure your paying through the nose whichever way you go. Ref biodiesel unless you have a supply of very cheap used chip oil, converting virgin cooking oil to biodiesel with the £2000.00 outlay for plant is not going to be viable for much longer. The price is going up and by the time you have paid for your oil, chemicals and electricity it's not really all that much cheaper. Any vehicle fitted with common rail injection is very intollerant of the fuel viscosity. You have to be careful what you put in these engines the wrong mix could cost you a fortune.