Loosing pressure ,ignition indicator flashing when boiler goes quiet?

VUW 242 1E loosing pressure when not running,cant find any leaks from outlet pipe,when the boiler goes quiet I.E. (not running heating or hot water ) ignition indicator starts flashing use on/off to reset.Also temperature gauge on zero at all times.I'm using it with the heating on all the time to keep the hot water going. HELP