The really effective way to remove sludge from a central heating system is to have it power flushed by a heating engineer. This involves connecting a special flushing machine into the primary circuit and pumping a flushing solution through the system for an hour or so.
At the end of the process you'll have a spotlessly clean pipe circuit and a reasonably big hole in your wallet (£150 to £250).
An alternative method is to buy a flushing agent - cost about £40 and add it to the system - you can get one that you squirt in via the air-bleed valve of a radiator.
Once the flushing liquid is in, run the heating system on a high thermostat setting for 4 or five hours non-stop. Turn everything off, drain the system, refill with clean water and drain again. you'll be amazed at the amount if filth that comes out when you drain down.
Now add a corrosion inhibitor (same way as for the flushing agent) and fill the system.
It will run much more smoothly, and the pipes will stay cleaner. The corrosion inhibitor will see to that. Ideally you should repeat the process every 18 months or so.