Detectors have to have a calibrated every 12 months, the recommendation for issuing tickets is 10%+2mph (so 79 in a 70).
Have they sent you a photo? it should show a copy of the callibration certificate.
Your cars speedometer must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph
Please excuse spelling/typos. Apart from being a clot it turns out I had one on my cerebellum that's now causing various problems!