boiler will not light?

I have a Biasi M110.32SM/E . Problem is cant ignite boiler in domestic or ch mode. Fan runs but nothing ignites. Removed sparking probe. It sparks for about 5 seconds when a tap is run. Switch tap off then back on and it will sprk again and so on. Gas valve seams to be operating. Stumped any advice would be gratfully received. MANY THANKS. MARTIN