One way you could make a slight gain on tax (in addition to the cheaper rate) is that mpg is generally less when using veggie oil. So, keep a record of mileage, show that your car averages 40mpg (instead of e.g. 35mpg on veggie).
Of course, I would have to say that if you can legally run your diesel car on 35p a litre of veggie oil (including duty) then you should be happy!!!
I posted a link recently, can't be a**ed to find it right now. But the gist was: raw veggie oil can gum up your injectors (think of the hardened spots of polymerised oil you get if you don't wipe cooking oil spatters off whilst fresh). So you need to add a solvent that will help disolve that. Also, valves wore out more quickly, you need to preheat the fuel (more viscuous at room temperature) and need a stronger fuel pump.
I would be interested to see if adding two-stroke oil would help the lubrication/solvent issues.
Diesel engines
can run on pretty much anything. IIRC Rudolph originally planned to use coal dust! My neighbour (who has experimented with cooking oil) once filled his Renault Kangoo up with petrol... managed to get a couple of gallons of diesel in too, once he noticed. He said the engine was a bit rough but he stopped frequently to put another gallon of diesel in and dilute the petrol, and after he had done a few hundred miles it was running as normal again.
Try that with a petrol engine running on diesel...
