My Potterton Suprima 50 keeps fires then immediately cuts out?

It worked fine before i went away for three weeks. Now i've come back i can't get any hot water at all and only twice have i managed to get the heating on by pressing the reset button.
I've checked there's water in the water in the tanks in the loft and they are fine.
I'm a novice in these things so any jelp would be greatly appreciated.
Many thanks