HMRC have recently revised the rules on the use of vegetable oil as fuel.
http://www.hmrc.gov.uk/briefs/excise-duty/brief4307.htm

Basically, as of 30/07/2007 anyone who uses less than 2,500 litres of vegetable oil per year will not need to pay any tax. We think this it's about time but do commend the decision and look forward to the difference it will make in encouraging people to think about using sustainable environmentally friendly fuels.