Andrew,
The wall switch controls which heating source you want to use, and has either Gas or Electric.
If you select Gas, it automatically starts the ignition proces and will fire up the Ultarstore providing you have removed the cowl cover from outside first, if not, it wil go to fail and not light.
Selecting Electric, you then have a master switch on the main control unit. You should find a series of 12v fuses, and some rocker switches, one of which will be the Water heater.
It is essential to ensure the mains switch is in the OFF position for the water heater until the tank is full of water, or you will damage the heating element.
It is always best to not connect the 240v to the van until after you have filled the water system to avoid problems.