Worcester 9.24 Strange Problems?

Hi, Can anyone help with my problem. My Heatslave 9.24 seems to have a mind of it's own. Sometimes it will not fire up, others it will run for a few hours on CH then shut down. The rads then go cold. The water pump and fan sound like thay are running, but no gas! - if I try and switch it on and off via the front switch or temp control, then I hear the click, but no gas! usually the same if I request DHW(but not always). If I turn the power off at the mains then usually it will fire straight up again - but sometimes takes five minutes of fiddling to make it work again. - This is seriously intermittent as it will work fine for a couple of days, then back to playing up again. - Please can anyone help. I am getting female grief as I never know whether it will fire up for her morning shower!!