Correct economical setting?

I have just moved to a small rented house, it has 5 Unidare storage heaters WMS718N and the rest are similar numbers, my electric bill has just come and for 2 mths is £234. What settings should the dials be on. I set them at 4 to start as advised on the heater but do not understand what setting the boost should be on. I thought the boost just used the stored heat, or does is use electric again during the day?