I genuinely can't remember making a trip up a certain dual carriageway one morning in April 2017 when my car was photographed by a police camera travelling at 38mph. I have requested photographic evidence and they have send me a picture of the car but there is no way to see who was driving. I have written back apologising for the apparent lack of cooperation, but letting the police know that I cannot confirm the driver as I simply cannot recall making that trip that morning and the photo does not identify the driver. They have since written back threatening me to provide the driver details or "face possible prosecution", adding that "a further notice has been recently sent under separate cover and this should be completed and signed as indicated. Please note that your failure to furnish the information in the manner required may lead to prosecution for that failure". As I genuinely can't remember taking that trip that morning I am not going to say it was me driving. What would you advise?