U.K. Hazchem vehicle signs

Does anyone know the regulations that apply to the display of Hazchem signs on vehicles in the U.K?

In my spare time I work with a group who stage public firework displays & have to transport van loads of these fireworks from our base to the display sites. We have been advised that the vehicles should show a '1.4G' hazchem diamond but, because we sometimes have to use hire vans or private cars, these cannot be permanent.

Can we get away with displaying a sign in the windscreen? I usually see similar signs on the back of the vehicles or sometimes on the side but does any regulation specify that the signs must be in a specific position?