Spot on, Con and Use regs allow for a 10% margin of error reading high. Think of it as a safety margin that could save you £60 and three points. Incidentally when I was a traffic cop nobody got done for less than 40mph in a 30mph limit because we exercised DISCRETION, we were not a bl**dy revenue making machine. Nowadays you hardly ever seen a traffic cop in London but plenty of yellow cameras, so much for road safety.