NO grey area at all adding kerosene is a no no and if you get dipped Kero is one of the things they sniff for.

For the sake of clarity and to prevent anyone from reciting hypothetical tales, urban myths and miraculous methods, to avoid been detected. Her Majesties Excise and Revenues Officers usually use an electronic sniffer that tests a sample of your fuel for certain trace chemicals that are added to Red diesel, Kerosene, Heating Oil etc etc. If the sniffer finds them it will tell the officer carrying out the test how many PPM (parts per million) it has found. If it is above the accepted level. You will have to pay duty not only on the fuel in the tank at the time. But could be required to pay all the duty on all the fuel you have used since you purchased the vehicle. The amount owed is calculated by using the mileage on the DVLA's database or MOT certificates divided by the manufacturers average fuel consumption figures. So if you've done 20k miles since you bought it you could get hit by a huge bill. Or they could just crush your vehicle instead..