Super tariff night storage heater not heating efficiently?

Hi. I have three Berry Magicoal night storage heaters which i inherited when i moved house. My energy provider is Npower & advised a super tariff supply which gives me 5 hours cheap electricity during the night & 2 hours during the day. This is ideal as it provides ample heating/hot water for my family. I don't have a manual for the heaters, but am currently finding them less efficient. The top half of the heater remains hot whilst the bottom half is cold/luke warm. I have the input dial set at maximum & the output dial at minimum. I'm aware of the function of these two dials & their role in how the heaters work. This has always been ok in the past. My heaters also have a circular dial on the righthand side, at the bottom, that can be turned...but i'm not sure what these dials are for. The dial is numbered 1-6 minimum to maximum & are currently set with the dial "pointer" on maximum! What does this dial do? And does anyone have any advice? All would be gratefully recieved.