The "switch" is in the supply network control room. The one that turns on thousands of heating elements when it activates the economy 7 switched supplies in thousands of homes. And yes there is a hazard with this.I find it strange and possibly dangerous that a main Room Heater can be controlled by a switch elsewhere in another room, it seems the heaters could be switched on/off, not knowing the conditions in a particular room. Sounds Odd to me.
With that comment then, what is the point of having switches for heaters in homes these days ??
IMO it would be advisable for the OP to get the heating and Immersion Heater Circuits checked out by a local Electrician ASAP.