The device (provided it is "type approved" and with or without a calibration certificate, regardless of what such a certificate shows) is assumed to be reliable unless the contrary is shown.
The burden to show it cannot be relied upon rests with the driver. Deficiencies in the calibration certificate, or even the absolute lack of one, may help in that task but, by itself, it is unlikely to convince a court that the device is unreliable.
Is it worth asking the police for the complete certificate/continuation sheets and underlying calibration results before deciding how to deal with the allegation?
You can ask for whatever you like but the only way to deal with the allegation immediately is to respond to the accompanying “request for driver’s details.” The police will then decide what to do. The most likely outcome is the driver will be offered a course for that speed (provided he or she is eligible). Failing that, a fixed penalty (£100 and 3 points) will be offered.
Taking advantage of either of those will require accepting the allegation as it stands. If the driver wants to get involved with technical defences, the police will almost certainly say the only way to do that (and so get to see the evidence they intend to rely on to secure a conviction) will be to decline any out-of-court solution and instead have the matter heard in court.