My heating has just turned itself on 
That's generally the way it works in these modern times.
If you hunt around, you'll come across a doo-hickey with a dial on or maybe a box with numbers.
These will most likely range from 16 to 25 or thereabouts. This the desired temperature of your hovel.
If the ambient temperature drops below this figure, your heating will turn on.
Usually this happens when it's colder outside as it is today like . .
All clear?