Gas combi boiler raditator flushing to clear dirt
Since there is no response to my question, I took a chance and tried as below and there was no problem, I have cleared all the sludge.

Turn off the Worcester gas boiler

Open the pressure valve of the boiler so that water flows into the system

Open the radiator drain valve to let the water flow until it is clear of sludge
Maybe not as efficient as power flushing but it is an easy and cheap procedure