Re: Calibration certificate, I believe that is only important for automated penalties. As you seem to say that you were caught by 2 uniformed police, the calibration does not apply, nor does the oft-quoted "10% plus 2". In addition, the officers do not need to prove they have been trained in using the device - they are uniformed police (not civilian operators) therefore they are deemed able to judge speed without the device, and it is only used to check their belief.
The "evidence" that will be presented in court is the testimony of Police Officer 1. The "corroboration evidence" that will be presented - as speeding offences must be corroborated - will be testimony of Police Officer 2.
There is something called a "NIP Wizard" that you can fill out on the pepipoo website, if there are any legal loopholes or defences from the NIP they will tell you about them. IIRC, Police issued citations are very rarely successfully challenged (as opposed to automated ones).
PePiPoo: Helping the motorist to get justice