As mentioned before...COMMUNITY SPEED WATCH... usually your local council type busy bodies. They buy the kit themselves, it should be calibrated but generally is not.
The letter isnt worth the stamp, ignore it and ignore the targeted bit its a ruse to make you worried. Perhaps if you had lots you may become an intelligence target, but for minor infringements I doubt it. You may get a visit from the local community cops to have a word in your shell like.
The vehicle ASBO is a different matter. Generally cops will witness an offence, wheel spins (being a complete nob), low level street racing from lights, doughnuts etc and you get an ASBO warning against the car or driver. IT stays live for 12 months on PNC and if you get another then your car is seized. £150 to get it back. This is a good tool for the boy racer Corsa crew. Complaints can also come in corroborated by 2 members of the public which could also be investigated and ASBO invoked.
Community Speed Watch runs only in a few areas of Scotland and lots in England/ Wales. Scotland is under review due to the calibration, training, risk elements and may well stop.
Having used ASBOs they are a great tool when used correctly for vehicles and the Friday/Saturday night crew.
Community Speed Watch can be a good deterrent for the moral majority, but for the enlightened its no great shakes.