Operating mode controls the amount of electrical heat going into the heat-storing bricks during the night. So 6 heats them up to their hottest overnight, but costs the most to run.
Room temperature controls flaps on the outside casing of the radiator that limit the amount of stored heat coming out. If the flaps are closed, least heat comes out but the heat already stored in there lasts longer before the bricks cool down.
One never 'loses' the heat stored inside these radiators - all one can do is control the rate at which it gets out. Bear in mind that the Economy 7 (or whatever the current name is) only ever puts more heat into the bricks at night - so once you have let it out, you can't get more heat from the thing until the following morning.