If you are confident you should be able to carry out this task. If you are not you should always ask a professional service like South Kensington Plumbers to do it for you. South Kensington Plumbers can advise on refilling systems.
Here is a basic guide to help:
Make sure that all of the radiator air bleed valves are shut. Now, shut the drain valve, and unscrew the jubilee clip that is holding the hose onto the -drain valve and take it off.

Make sure that the hose is held upwards so that any water left inside will be able to drain outdoors. Always take the hose outside to roll it up before you store it away. A South Kensington Plumber has the tools to do this job.
Next, open the inlet valve for the feed and expansion cistern so that system can start to fill again. Inspect all the radiators, and then, open the bleed valve on each, and release any air that is inside. You should do this first for the downstairs, then you can do the upstairs radiators. This should make sure that air cant re-enter the system. When all the air has been released from the system, you need to return and check there is water in the feed and expansion tank and that this is at its usual cold level. When it is cold, the level is low as there must to be room for the expansion of the water as it heats. You now need to visually check for leaks if you have been working on the system. Switch the boiler back on, using the manufacturers instruction book allowing the heating to run up. 

NOTE:
There will be a requirement to bleed the radiators again as it heats up, and there may be a need for this over a couple of days as the system settles.