I have seen ads on a bus near oxford about cameras which can catch tax dodgers?
Anyone know anything about these?

guessing every time you drive past a camera it take a pic of you No. plate, that is then read in to a database & cross referenced against the DVLA showing who has car tax.

thats how they say the london rip off charge is reported to work

I have seen the vans out and about that are there to catch tax dodgers. Basically they scan the number plate, check if it is taxed or not and send out the ticket if not taxed.

Presumably the slight snag with this is that they need to actually have the correct details for the registered keeper, and what are the chances that these days with more and more speed scameras more and more people aren't registering their cars?

This may be why more recently the tax cameras seem to work in league with the police, and either they are doing vehicle checks at the same time, or they have a police car parked just down the road.

Chris