Is that the only picture they sent you - it doesn't even show the road, let alone markings!?

I suspect the camera inside the van is fixed firmly in one position aiming exactly where they want it to, thus it would be quite unlikely it is targeting the road outside the 30 zone -but you never know!

With the van being parked outside the 30, and the photo being quite inconclusive as to your position on the road i'd be sorely tempted to fight it myself...

If you go back and take a few pictures of the road from the approximate position of their van you should be better placed to make a call on whether there is enough doubt to contest it?

Tom