Boiler pilot system Ferroli Combi 77 FF?

boiler not firing up after proper ignition sequence is followed, pilot extinguishing itself after ignition button is released. heat sensor which senses pilot flame looks damaged, have cleaned it with emery paper but to no avail. also lots of white powdery build up inside the burner chamber on heat exchangers etc. looks like leak but no sight of water, pressure also drops on pressure guage and needs frequent topping up, any ideas