i meant to say 25 July i got flashed not August.

Hi, I am a new taxi driver, i bought a used (05reg) car for taxi over a month ago, i had it M.O.T (taxi Mot) and started driving approx 3 weeks ago. Anyway i was flashed by a camera about 2 weeks ago(25th Aug), the NIP came to the old owner a couple days ago who will be sending my details off. the problem is that i was 100% sure i was driving at 30mph, but still got flashed...iv never had the speedometer flicker or stop working, its just worked as it should without problem during the cple weeks i driven before being flashed and even now, therefore nothing to suspect. But i was advised by a friend so decided to check my speed and found that the when the speedometer showed i was driving at 30mph the car was actually doing approx 36mph. as i explained before i just bought the car, had mot done, never had the speed meter flicker or give me trouble, but little to my knowing it was always showing me wrong speed. now am a very patient and good driver. but feel that i will be given points and fined for something that was undetectable. i am really worried as the points would make my insurance jump. Please give me any advise on how i can prove my innocence. if it had been an obvious problem with the speedometer like it flickering or playing up then i would admit it was my fault for not getting it fixed, but in this case there was no way of knowing. I would appreciate any help and guidance to having this resolved, i dont want to go down without trying to prove my innocence. Thankyou