Thanks for the feedback, just to clear up a couple of things I wasn’t clear about originally.
The first letter I received was for a car which was SORN, it was at home in my garage and I was at work at the time. I contacted the local police force where it was reported as I was worried about it having been cloned. The police said it was more than likely a mistake but they would keep an eye out for the plate.
The second letter was sent to me for a different car, this one was dated the same date a girlfriends birthday, I saw the speed watch team as soon as I came round the corner at around 15mph, I sat at around 25mph while I passed them.
The third letter was last dated last Saturday, again I saw the team, one of them register my speed, I checked my speed which was 27MPH. I saw them write something in a book, I’m guessing my reg and them today I received a letter. I remember wondering why they had bothered to write my reg down when I wasn’t speeding at the time. There was a car behind me which caught me quickly coming through the 30 limit on this occasion so I wonder if they recorded the wrong car, having never used a speed gun I don’t really know if this is possible.