How does my Horstmann Economy 7 Quartz/Maxistore control work?

I have the above control box for my economy seven electricity and my bill seemed very high for Aug to Nov. Looking at the device underneath there is a white switch with a red light that comes on when switched on and is labelled 'Water Heater' what does it mean is this is perminatly switched on?

I unfortunatly have no one I can ask to come and look except without paying a fortune so any help would be great.

Thanks