There have been several cases of people being flashed for under the limit. If they try to prosecute you can ask to see the evidence, you can then measure the distance travelled in half a second using the 2 pictures with the markings on the road. From that you can calculate your speed. If the speed turns out to be less than 31 you can plead not guilty. In reality they will drop it if you demonstrate that you where not speeding. This happenned last year to a van driver who, like you was sure he wasn't speeding, he calculated his speed as 17mph! Good Luck