The notes on the back of the V11 say that "If you do not relicense or make a SORN declaration you could be fined up to a THOUSAND pounds(!) or 5 times the annual rate of duty,whichever is greater."
The sad thing is that the folks getting fined are people such as yourself and/or have simply forgotten to tax the vehicle.They dont catch the gits that never tax a vehicle from day one,along with MOT's insureance etc.
