As Russ said the police usually do 2-3Mph + 10%, so you'd have to be doing 35+ for a copper to take action...
BUT!!!
The police can do you for anything over the posted speed imit
and
Most gatso's are set to 10Mph+ over the speed limit at least
You sure it wasn't flashing an oncoming car or an overtaking car?
Anyway if a NIP isn't recieved within 14 days you're OK...If you do get one though at 31Mph you are over the speed limit
