Hi Jimmy, Recently I was in the same position as yourself.
Family trip down to alton towers, 2 cars going down, frequent stops at service stations, multiple drivers, was a front on photograph, caught my car at 89 in a 70 on the M6. I asked for photographs which didnt show the driver clearly. Sent letter to the police camera unit saying i could not identify driver from the stills provided.
However i Could supply a list of possible drivers it could have been. The camera unit then passed my case to the CPS and a summons was issued for failing to provide identity of driver. I pleaded not guilty and case was ajorned to end of May for my trial date. In the meantime I had written a letter to the CPS asking them to look into my case and asking if there was anything else i could have possibly done to identify the driver and on the evidence provided that no one could be 100% certain as to who the driver was. I supplied them with my defence argument i.e my corresondence with the Camera unit and also my defence argument(parapgraph 4 defence for Section 172). The cps dropped the case saying there was not enough evidence to guarantee a prosecution and awarded me costs. Was nerve wracking but a good result in the end.
My advice to you, is to send all correspondence by mail(special delivery) and to keep photocopies, so if the matter does goto court then you can produce your letters showing your efforts in trying to accertain the driver of the vehicle and ur willingness to help to identify. Good luck.