I have never understood people being prosecuted at 33 in a 30 limit, no more than I understand claims of zero tolerance in certain areas (such as North Wales).
Tell me about it

I was done for 33mph!
Many years ago I was driving thru Llangollen(30mph zone) going towards Bala in my Ariel Atom, saw a national speed limit sign ahead fast approaching, and just as I was about to put my foot down i noticed out of the corner of my eyes there was a van parked up behind the bushes literally just in front of the National speed limit sign(very sneaky), I quickly glanced down at my speedo it was indicated at around 35mph, thought I got away with it and of course I got the letter a few days later.
I bet they must have caught many out on that day!!!