A filling loop should be fitted to your system and should be positioned very near the boiler. It will usually comprise of a 'butterfly' tap connecting to a flexible pipe which connects via a non return valve to one of the boiler pipes. Once located ensure the boiler is switched off, open the butterfly valve and the system should start to pressurise. Once upto pressure close valve and retry boiler. If you cant locate filling loopor are unsure check with a couple of plumbers to find what they would charge to repressurise a system. They will then also be able to demonstrate what to do in future & if this does not cure the problem they are already on site to diagnose it.
Robin S
August 2008