I have this heater and posted below your question as mine is broken at the moment!
Anyway, basically the input dial controls how much heat is generated and stored inside the bricks. The output dial specifies how much of that heat to let out the following day. Try setting the input to 5 and output to 4 and see how that suits your room (output must be set lower than input or it will let out more heat that it has!). There is no light to tell you it is on by the way, but there should be a switch on the wall to turn it on/off. Economy 7 will do the rest when midnight arrives.
Jamie Moore, November 2007