The idea is that it heats up at night, using cheap off-peak electricity. It has great lumps of brick or concrete inside, or a lot of stone chippings. These store the night-time heat, which is let out slowly the next day.
There are usually two controls. One sets the input temperature for the night time. The other controls a vent to let the heat out when you need it. You'd normally turn the second one down at night, and up in the evening, while leaving the first one set to the same amount all the time.
They are not bad if you spend the day in that room. However, they are pointless in a bedroom, or if you are out all day -- by evening they're often cool again. Not much cop if the weather changes suddenly either. Often better to have a fan heater or convector.
One thing to check is whether the clock on your off-peak meter is set correctly (or indeed if you are actually still on an off-peak tarrif). Sometimes the timer gets out of sync (from power cuts), and the input happens in the daytime -- this can be quite convenient, as you pay off-peak prices at that time instead. Of course, one really ought to tell the leccy straight away.