"1. Surely the mobile speed cameras should not be recording my vehicle in the first place if I am not speeding.
2. How can they prove actual spacing from a photograph"
1. - If you think about it, they must be recording all the time. It wouldn't be possible to aim the speed gun at your car, record the speed and then turn on the camera.
2. - They can blow up the picture? Quite easy I would have thought.
Also bear in mind that it could be the policeman's observation that your plate was illegal - in his opinion. Doesn't necessarily need to be captured on film.