To answer this properly, I am going to have to explain how a storage radiator works.
It is like a huge electric fire, with the heat energy stored in bricks inside, and the casing well-insulated (to stop the heat escaping too fast into the room). The fire heats up at night when electricity is cheap to buy, and it lets it out during the day - slowly (when electricity costs about 3x more than it does at night).
It is usually best to turn the 'input' control up the maximum - that way you get the maximum heat stored inside when it is cheaper to buy the heat. The 'output' button normally controls flaps in the insulated casing. With the flaps open, more of the stored heat escapes into the room - so more heat output - but of course the whole radiator cools down quicker. Once it is cool inside, that's it - no more cheap heat available until the following evening.
So the generic answer to your question is, turn the input up to the maximum, then control the output so it is as low as you want to supply a reasonable level of heat into the room by day (keep it closed at night).
What the boost button typically does is to connect the fire input back up to the standard tariff electricity supply by day, when it gobbles up electricity costing 3x the night rate. Avoid using this at all unless you have to.
The fan will just blow air over the hot bricks inside - increasing the rate of heat output.