UPDATE :
After taking advice I have written to the safety camera unit stating that my car was locked on my drive with the keys locked in my house & that no one had access to the car at the time of the alleged offence & that I do not believe that the car in their photo is mine.
I included photo"s of my car as proof.
I did not return the S172 form as I cannot legally put any details down on it as I was not the driver & I do not know who the driver was as it was not my car.
I have informed DVLA & the Police that I believe that my car has been cloned & included that information along with the ref numbers the Police gave me in the letter.
I am now waiting for a reply from the safety camera unit.
I will be surrendering my registration to the DVLA & getting a new one.