You should have a markets licensing policy in place before attempting any action to control a car boot sale. However, if the car boot sale has been operating for some time then you are unlikely to be in position to take effective action. The ultimate enforcement action to protect market rights is to obtain an injunction but the grant of an injunction is a discretionary remedy and if the court feels that you have delayed taking action it is unlikely to be sympathetic. You will have to demonstrate that there has been some material change in the car boot sale operation such as an increase in size.