I hope you haven't said you were driving? What they say is don't initiate correspondence. Wait for them to apply for the details of the Registered Keeper and write to them. They need to be able to prove that the driver entered into a contract by parking there. They may tell you its down to the RK to provide details of who was driving or its down to the RK to cough up - its not.
If you ignore the letters which may look official they will go away after 5 or 6. You may get letters from Solicitors threatening all sorts, CCJ's, court costs ect. Its all an act to get your money. Stay tough and don't worry about them.
Look at this site here. Either of the top two posts will be relevent to you.
http://www.consumerac...ing-traffic-offences/