My combi boiler is heating the radiators when the central heating isn't on?

British Gas have told me I need to Powerflush my combi boiler as the radiators are turning on when the system is running and heating water for the taps.
Is this the case or is it a gimmick that once I have the system flushed they will charge me for something else?
If the system suffers from scale build up how does it affect the central heating radiator pipes?
Thanks all!