5. Bleeding the radiators

Usually when a central heating system is filled for the first time, pockets of air get trapped. It is necessary to bleed the air out, otherwise circulation gets disrupted and radiators may fail to heat up. Every radiator has a bleed valve at the top, and if you open the valve with a key (available from any builders' merchant) you can release the trapped air.
XXYou may need to bleed again a few days later because when water is heated up, any dissolved oxygen it contains is driven out in the form of tiny bubbles and of course is again trapped at the top of the radiator.
XXFresh water is rich in dissolved oxygen and is highly corrosive to steel so you should avoid draining and refilling your system once it has been commissioned. It is also good practice to use a corrosion inhibitor such as "Fernox." Simply introduce it into the primary circuit through the Feed/Expansion tank in accordance with the manufacturer's instructions.