Hi Rachel, if you look under your boiler you will see a large pipe on the left. Follow this pipe and you should see a junction where a smaller pipe is connected, this pipe will have a silver coloured valve. There should be a flexible pipe (usually covered with a metal mesh) this should be connected to a valve with a lever on it turn this lever slowly and you should hear the sound of water going through the pipe. Watch the pressure guage and let the pressure go upto 1.2 bars then close the lever. Job done...
But more importantly do you know why your system has lost pressure?
Mick
July 2008