Most plumbers will run the powerflush as Gandtea says.
However, to do a really thorough job, the entire system is powerflushed, emptied, then the radiators are removed, taken outside and powerflushed independently!
That's a huge job though, so most often it's just the ordinary powerflush!
We had the 'major' job done when we changed from a back boiler to a combi, the difference it made to the speed the heat got around the house was astonishing!
We still have our DS's old terry nappies (Ashton's Zorbit Gold, you just can't wear them out) so they were used as dribble cloths (not for the first time either)!