the presure should be just over one bar when the system is cold
there should be an external fill loop just under the boiler depending on who instaled it it is usualy a flexible pipe withy a tap on ether one or both ends
when you blled a radiator you must top up the presure at the same time as bleed a rad wil cause the presure guage to fall
www.gascentalheatinguk.info, January 2009