I believe that by making matters so unclear, Revenue and Customs are takinng the choice away from those who would prefer to run on biodiesel.
Try finding out about it. It's nigh on impossible to find a straightforward answer on HMRC website. There's lots of information about SELLING biodiesel but I couldn't find anything about the tax position of those who use it. Even other websites that advise on the use of veg oil seem confusing (probably because they're confused themselves)!
I've known someone have their car seized and left stranded by the tax police on the A1 literally hundreds of miles away from home for running his diesel car on veg oil.
Why make it so hard for people to use a much more environmentally friendly fuel in their cars?