ACPO guidelines is 10% plus 2 which is the speed they enforce at i.e. 68 in a 60 (not 69 as commonly beleived)

i had this arguement when i got caught at 46 in a 40, i spoke to the scamera partnership and said about the 10% plus 2 so i thought they should enforce at 47 in a 40, they replied with "we enforce at 10% plus 2 right on that limit which is 46 in a 40"

some constabularies and scamera partnerships choose to ignore this and will prosecute at 1 or 2mph over the limit. but this is a minority. if you went to court (instead of accepting a fixed penalty) over a 1 or 2mph over the limit thing you would probably get off with it - depending on the judge's mood.

some info here:

http://forums.pepipoo.com/lofiversio...php/t3770.html