Flame on for 2 secs. off for two hours. !!!!!?

On my Vaillant boiler, which must be an old model as it has no l.e.d. readouts etc, the flames will work for about two seconds then stops for two hours or two days... No matter what buttons I presss it will not come on unless it wants to. Once it came on in the middle of the night. Once it wouldnt work for four days. Flames won't work for heating or for hot water, except for two seconds. My children are cold and dirty. !!!Engineer says I shouldnt have bought a Vaillant and he can't fix it unless I can get him the technical help phone number. Does any one know it? Or can you diagnose problem?