Diesel fuel is heavy oil which does not vapourise easily at low temperatures. Vegetable oil as bought straight from the supermarket can be used as a fuel for compression ignition engines but it is a bit too thick and gloopy to be used in a vehicle diesel engine. It needs to be thinned down, and the easiest way to do this is by mixing it with regular diesel, something like 70/30 diesel/veg oil. Failure to do this will result in difficulty in cold starting. Some people say that the lubrication qualities are superior to regular diesel, especially modern low sulphur fuel, and so claim it's better for the pump, injectors and emmisions. Diesel we buy at the pump is filtered to about 25 microns, and so if you're thinking of using old and used vegetable oil make sure you filter it very well.

Also remember that you are not paying fuel duty on vegetable oil used as road fuel, but to declare the duty to HM Customs and Excise then makes it perfectly legal to use.

less than 2500 litres - HMRC are not interested