Emails are not merely dated, they are timed. So that should help. But is there any possibility you were on the A3 on the days surrounding the 21st December? A clerical error does not get you off. But proof that you were nowhere near the A3 not just on the day but on the surrounding days (or weeks) may be needed if you challenge the NIP and the case goes to court.

Dave emailed back on 5th January:

Many thanks for your advice re: alleged speeding in Surrey ion the A3 when I was actually at home in Kent. Having submitted the evidence of dated and timed emails that I had sent on the day in question, I got the following answer from Surrey police: 'In view of your comments the photographic evidence has been reviewed and you are advised that your vehicle had been recorded in error and was not the subject of the incident. You may be assured that you have no liability in this matter and we offer sincere apologies for the inconvenience you have been caused. Yours sincerely, Surrey Safety Camera Partnership".