Ok,
Thinking about it and watching the video from watchdog I am tempted to try the following.
Dear Sir/Madam,
I received a parking ticket on **/**/** but I will not be paying your demand for payment as I was not the driver of the vehicle at the time in question. I am also under no obligation to disclose who was driving the car so please do not contact me again with regards this matter.
Yours Thankfully,
That letter is actually copied from the letter on moneysavingexpert. There is another letter on there to use if you continue to get letter so I might see how far this goes.