I used to do this years ago - prob is now that veg isn't far off diesel price, and when you pay your duty (as one must) it becomes an expensive alternative, unless you have access to plentiful cheap used oil. toyed again recently as I got a bookers card through the business - think you can run 2000 l of bio before you hit the duty limit. A good start would be just to add a hearer to the fuel line and filter and mix a blend 50/50.