I will answer this now I have access to the site back.
There are a number of ways a person can be reported for excess speed.
1. On an A class road, you must be followed at a constant distance for a minimum of 3/10 of a mile.
2. On a Motorway the check must be carried over a minimum distance of 1 mile In both cases the speed is measured against a calibrated speedo which is checked at least once a week for its continuing accuracy.
Traffic cars only have the calibrated speedo, area cars and panda cars do not, so whilst they can do following speed checks, their speedometer has to be checked as soon after as possible and the calibration details recorded as part of their evidence.
3. VASCAR which is an onboard computer which measures time over distance and gives an average speed.
4. Truvello which are 2 cables laid across the carriageway a set distance apart and record vehicle speeds (again time over distance but in a shorter range) and is usually used for speed monitoring but can also be used for enforcement.
5. Lazer which is the most commonly known which are hand held speed recording devices which can measure speeds of up to a mile away with absolute accuracy.
The last method is a lesser known offence which is called speed reckless where a Traffic Policeman can give evidence of opinion where they have not for whatever reason been able to use conventional speed recording devices and so they are able to report someone for the offence based solely on their evidence, but it is very rarely used and in all my service I think I used it twice, but it was coupled with other offences as well.
There are national guidelines regarding at what speeds people get reported, but by law you have to be given 10% because that is the margin built into speedometers by the manufacturers.so for example 33 in a 30, 44 in a 40.
In some forces such as my old force 12 MPH is allowed before anyone is reported on the basis that at 42 in a 40, you know you are banged to rights.
On the Motorway, drivers are not reported usually until they get to 90 and above, but if someone gets mouth for doing 80, then the option to report still remains.
So in your case, don't worry, if they were concerned, they would have stopped you and reported you on the spot, but to judge your speed as being just over the 20 would be difficult and I know of no Police vehicle that records excess speed from the rear for a whole number of legal, logistical and practical reasons.
