pilot ok ,fan ok, pressure ok but no hot water?

morning all
am having probs with my swift flow 80.i had a problem with it holding pressure but after it spit out a big lump of muck out of the overflow it seemed ok for a few days .now even though i have a good pilot light the fans ok and the pressure is at 1.5 bar it only ignights every few times i turn on the tap the rest of the time it just runs cold help