Understand continual pressure loss is caused by leak in the central heating system. However despite four seperate visual inspections no evidence of a leak can be found. Cold pressure test 2.5 bar has failed to locate problem. Combi tested for one hour seperately under same pressure was satisfactory. Boiler installed January 1999. Lower floor area isolated and held normal pressure for 2-3 days but unfortunatley is now losing pressure. Suggestions please!! Assistance would be gratefully recieved.