It is actually impossible for a mechanically driven speedo to be 100% accurate.
They do not have to allow any margin this is a recommendation, and thankfully one which most courts uphold, the offence has been commited even if by only 1 mph, why it was commited is another story, should all people with sat navs be treated different as this tells them a more accurate speed then ?
Most forces have thier speedos calibtrated by RS in Nottingham, and even RS do NOT gaurantee 100% accuracy as it is impossible, they can gaurantee 100% accuracy from input but this is the problem, the input(speedo drive) can never be(100%) accurate.
tabetha