Point blank refuse to pay and demand that they take the case to court where you will defend yourself with the evidence that you could not possibly have been the driver of the car at the time. However, if they can prove that the photographed car was in fact your car, as the registered keeper of it you will have to tell them who was driving at the time or take the rap yourself. You need a solicitor to fight this. If you win, you can the sue Merton Council for your costs. Authorities need to learn that cloning is easy and convicting registered keepers of cars by registrations is not reliable so they need to allow for this.