Most people will try and do this themselves with no problem, but you need to be confident in your ability or call a professional like Croydon Plumbers who can drain a heating system and also will give a guarantee for the work that they have done. Any time that a fault occurs in a central heating system is not normally an easy or quick or maybe cheap fix. As it involves hot water under pressure, the scope for disaster is immense. Therefore it will more than likely need to be isolated and drained of all water to fix the snag (providing you know what and where it is).Guide:1). Initially you first action will be to isolate the power for the boiler, at the boiler itself, you should isolate the breaker or remove the fuse as well as the pump can sometimes remain live. Turn off the supply to the feed and expansion tank.2). In the loft you will need to use a wooden batten and a cord tied in the middle. Lay it across the top of the tank and tie the cord around the arm of the float valve keeping it raised up (or closed). Push a rubber hose on the drain of the tank and run it out to a main drain, making sure the flow is at least on the level but never uphill as this stops the flow. Open up the valve allowing the tank to drain, have some cloths nearby just in case. A Croydon Plumber carries all the correct tools to carry out this task. Check the hose and connections for any leaks.Once the system is empty, carry out the repair. When completed, close the system again and remove the batten and open the main stopcock. The radiators will need to be bled. Finally:Dont forget the pump because it can airlock too.