For a definite answer we'll have to wait until Gasman logs on. Come on Gasman where are you when we need you?
However I have thought about it more. Surely there will be very little difference in cost. If the house is kept at the same temperature then the same amount of energy is put in, just in shorter higher bursts when the boiler is set high.
However with the temperature set low the water pump will wear out quicker as it is running for longer. With the temperature set high the bolier will suffer wear quicker as it runs at higher temperatures. (Or will this stop it coking up?)
OK so I don't know.