Don't worry about it. If it was a 70 limit and you KNOW that for sure, any NIP will give the speed which, you say, will be below 70, and what is claime4d to be the limit you are alleged to have broken, so you can contest the claim by writing to state that the alleged limit was incorrect on the date noted.
If you have made an error and the limit was in fact 50, then 3 points and £100 penalty are on the way.
Bl***y cash-cameras making no difference to road safety at all.
