your pilot flame heats a thermo couple and this generates a tiny electrical current [ 27millivolts]that energises a magnetic solenoid that hold the gas valve open when you release the button,is the pilot flame touching the thermo couple tip [ a round thing about 5mm in diameter with a cappilary tube about 2mm in diam coming from it] the odds are its not so the gas jet needs cleaning to give a nice hot flame ,If it is heating it the thermo couple has packed up [b&q or plumbers merchants about £ 5.00] if this does not work the solenoid has most likely gone, another thing that can happen is the button you are pressing isn't quite pushing all the way in so the solenoid is energising but the valve isn't touching it so the magnet can't hold it open.straight forward job but remember gas can go bang and a gas fitter is a better bet' but at least you know what the problem is and how it works so he can't baffle you with science
gasman 46, October 2008
