sabre boiler wont light?

we have a 2 yr old sabre boiler heating is on all the time at the moment. problem is it just wont light ,when dial on water only ,turn tap on and water runs cold and light on boiler goes to red the same if on water and heating put it on timer and it just wont light,the light just goes to red .2 plumers have looked at it got it lit, said its fine water and heating all ok ,but 20 min later red light back on no hot water or heating.managed to light it after about 20 min of trying and left it .any ideas please