Looks like a case of find your local biodiesel supplier buy a tankful and keep the receipt in your car. When you get pulled by the plod present the biodiesel receipt as 'evidence' that the car contains legitimate tax paid chip fat, drive away. Many biodiesel outlets are selling 5% mixes (5% vegetable oil, 95% derv) but there are an increasing amount selling pure biodiesel which presumably would smell just as bad (or good depending on what you're opinion is) as regular cooking oil unless they're treating it in some way.
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Regards, Jason.