On a combi boiler, you need to refill the cent heat using the filler loop located (usually) below the boiler.  You may have to attach pipe to do this.  You simply open the valve, and mains pressure water will then enter the system.  You need to keep an eye on the preesure guage and fill to approx 1.5bar for most systems.

Bear in mind you will loose some inhibitor you may have in the system.

I often simply isolate the rad at the lockshield and thermostatic rad valves wither side of the rad, and then slacken the two nuts to allow the rad to be lifted and tilted forward to be rested on something flat while you remove and re-attach paper.  The rad then only looses a little bit of water as you slacken and re-tighten the nuts.

Thanks for the help guys. The boiler is a Baxi 105e and it has a silver braided hose with a valve situated underneath the boiler, which I think may be the filler control. The idea of not draining  the radiators down completely is great, I'll do that. Thanks also for the tip about the mounting brackets. Our bathroom radiator (not one of the ones I need to decorate around) is very noisy so hopefully I can fix that while I'm at it.

cheers

George