Alasdair It's not law AFAIK, and it's also immensely unlikely to cause an explosion.
Think about it: you have cars with many electrical contacts and a HT system *specifically* designed to create sparks all around you- all much more likely a source of ignition than your phone. Still, the attendant is just doing what they have been told by the owners, so it's really not worth the bother: I'm sure that call could wait :-).
The Mythbusters TV program (OK, I know it's an entertainment show, but their tests were scientific) disproved this one. They experimented with the perfect fuel-air ratio, and placed a mobile phone in a sealed cabinet of fuel and air, and rang it several times. Nothing happened. They then did mmanage an explosion by using a van der graaf generator, IIRC.
I think it's a case of urban myth and over-caution, though this is all based on that program, so if anyone knows better, shoot me down.
Petrol station fires are apparently much more common in the US, and one study found it was due to the american practice of leaving the tank filling (unlike our pumps, they can be latched on), and going for a nice sit down it the car, getting a static charge, and then reaching for the nozzle after getting out- causing a spark to jump to the body of the car in the vicinity of the fuel vapour leaving the tank.
