sneaky speed scamra's.

[BBROWN1664]

I have never heard of that 10%+2 thing before.

Several years ago the association of chief police officers decided that +10%+2mph was the minimum that they would do people for to avoid clogging up the system with people that were only 1mph over the limit.
Theoretically all cameras would be set to use this threshold (46mph in a 40mph, 79 in a 70 etc) so that the films would last longer too.
Some police forces ignored it (North Wales) and others go a bit higher.

Your speedo is legally allowed to read anything between 70 and 77mph at a true 70mph. If the cameras go off at 79+ your speed could be showing anything from 79-89mph (allowing for allowed errors) meaning you have no excuses.