my boiler wont work it sounds like a kettle?

i have a valliant boiler the pressure dropped so filled it back up it was working fine then the pressure dropped again i have filled it up again bled all the radiators and its not working it fires the pilot light, sounds like a kettle taking of then goes offthe numbers on the panel rise rapidly as if its reaching the temperature then goes off it does throw in a a fault sometimes f20 and the pump gets very hot can someone help please.