If you wish to challenge a speeding ticket then you cannot do this via the police in any circumstance - instead, you have to go to court and the court has to agree that your grounds to contest are fair and correct. You do have a right to see the calibration certificate and any photographic evidence but you must request this through the court system. If the force responsible cannot produce the evidence or the calibration certificate then the conviction may well be overturned, but the flip side is that if the conviction is deemed to be sound, you may well find yourself liable for court fees and a fine on top of the original fixed penalty. We would advise discussing your case with a specialist motoring lawyer before going any further as legal advice is beyond our remit, other than to tell you that you have the right to appoint a appoint a third party legal representative to assist you.