If you are using bio (or any other alternative fuel) thats not duty paid, you are required by law to record how much you use.
In the (unlikely) event that you are randomly dipped, you will need to prove that you haven't used over 2500 litres in a year. If you can't prove it, HMRC could assume that all your yearly mileage has been done on bio, and calculate duty owed based on your car records (MOT/insurance etc).
Mark
Luckily enough I do about 10000 miles per year. At 25mpg thats just under 1800litres! I will never have to pay!! Ha ha!!