Ariston. Can't find manual... Turn on hot water tap, fan runs, pilot ignites and dies after 10 - 20 secs, thus no hot water. No pressure in system according to gauge. Have checked rads and external "overflow" pipe for leaks, nothing. There is a plastic tap on flexible metal pipe under boiler that I have turned thinking this will allow water flow and pressure to rise again and system to work... nothing happens. Any suggestions as to problem please ? 
