What can I do to protect my number plate from cloning?

I have just received a letter from a car parking enforcement company, telling me that a car with my plate had overstayed at a Sainsbury's car park (no fine this time). This was in Stoke-on-Trent on 19...

Cloned stranger

I am the owner of a Mercedes Benz SL55 AMG with a very special registration in Designo Silver. It would appear that a blue MB CL500 is using my registration. I have a letter from the Metropolitan Police...