When you have the system drained down you can do a mains water flush quite easily yourself using the filling loop as the source of the mains water.
The filling loop in most cases sends water into the return side of the CH system, I’m assuming this for these instructions, if the filling loop is connected to the flow side of the system use the return pipe as the drain point!
If you have a drain cock anywhere on the flow pipework use that as the drain point, if not you need to choose a radiator that is in a convenient location so you can run a hosepipe to a nearby drain.
Make sure to make a note of how many turns open each lock shield valve is set to (Return side normally) so you can put them back to where they were when you are finished.
Also if you have a manual bypass valve make sure this is closed but once again take note of its position so you can put it back to where it was when you are finished.
Disconnect the valve on the flow side of the chosen radiator, now you need to connect the drain hose to this valve, you may have to cobble a fitting or two together to connect to the valve, alternatively if the pipe to the valve is 15mm remove the valve & then you can push the hose straight onto the pipe, securing it with a jubilee clip.
Remember to close the return valve at the other end of the chosen radiator.
Close the isolation valves on the flow & return pipes into the boiler, (the ones just under the boiler). Now go round all the radiators and close off both valves on each, make sure the bleed screw is closed on every radiator also.
Open both valves on one radiator, the one you want to flush.
Now open the filling loop as if you were filling the system... fresh water will go through the return pipework into the rad you have open & come down the flow pipe & come out where you have the hose connected, if you watch the water coming out of the hose at the drain you will see when it runs clear, when it does that radiator is flushed.
Turn off the filling loop & close the valves to the radiator you have just flushed & move on to the next, open the valves to it, go back to the filling loop & open it just like before.....carry on like this until you have done every radiator.
When you have done them all open both valves on all the radiators and open the bleed screws to fully drain the system, not forgetting to open the isolation valves on the flow & return pipes into the boiler.
That's it done a mains water flush.
You will not be flushing the whole radiator using this method as the radiator won't fill to the top with water, as the bleed screw is closed, however the water will be forced through the radiator along the bottom section, the area that is prone to any build up of sludge etc.
If you have a heavily sludged system it is advisable to remove each radiator & take them outside to flush them with a hose pipe.
I would do a flush like this to get any thick sludge out of the system, then add a cleaner like Fernox F3, this can be left in the system for a week or so while you use the CH as normal. Then drain the cleaner out as per recommendations on the bottle, then do another flush as above.
Once this is done fill the system making sure to add an inhibitor. (Fernox F1)
Check out ebay for some good deals on Fernox F3 cleaner & Fernox F1 inhibitor.
Hope this helps.
Cheers.
