Valiant boiler hot water?

i have just moved into a new property and as you can imagine a little low on funds to call out a heating engineer. My heating keeps turning itself off gas is coming through as you can see it burning but it does not fire itself up first time also it turns itself off after a matter of minutes so we can only get hot water for a couple of minutes at a time. Does anyone know what the problem is and how I can fix it? Please help as iam getting nothing but ear ach of the mrs.