You may find my answer a bit odd, do you know any HGV Haulage firms around your area? you need a firm with a MOT testing Station, within these stations they have what they call a Rolling Road this is as said a Rolling road that Calibrates HGVs speed, Each time a Lorry comes in for it's test the Taco is Calibrated to the Machine with a printout, If you would be prepared to ask with the cost to Calibrate your car you can then decide whether you want to challenge the speed the police state you were going at, also ask for the police calibration of their device this they have to do & also issue if asked, I do not condone speeding but we all make mistakes (Even the police speed at times but who are we to argue).