If they do not have a radar reading then take it to court. A surprising number do not stand up in court where evidence must be produced.
My mate (an IAM rider incidentaly) was recently prosecuted for speeding on the evidence of a guy in a panda driving the opposite way who thought he 'must' have been speeding as it took 2 miles to catch up.
It did not stand up to scrutiny in court. I would get a good soliciter and fight it.
I have first hand experience of good and bad policemen so I know both types are out there.