I haven't looked at this for a year or two, but the bottom line was that you are suppoesd to pya duty on it. If you bought veggie oil produced by a certified supplier (not sure if there are any now) then your receipt showed your paid fuel duty.
If you were just doing it yourself then you were supposed to keep your receipts etc and pay up at some stage.
I suspect many people just put the veggie oil in.
Do you mind me asking what car/engine you have and what fuel you are using?h