We had a spate of them go missing from vehicles recently, which led to the instruction not to leave them on the vehicles when they were not being used. Which I guess is the point of mag mounts really.As a result of the theft issue (and the way that it scares the bejeebers out of you when magmounts fall off your car when you are going along) and of course the aluminium roof of the Defender, I favour internally mounting LED lightheads in the windows to give 360-degree cover - and as LED lightheads can be picked up for not much more than £30, can work out cheaper than a decent quality lightbar.
Cheers
Blippie