Valliant boiler making noises?

Our vallient boiler has, for a while, been cutting out with either error code f22 or f24 (don't know the model, can't find it anywhere on the boiler). The pressure guage reads normal. We have been making use of our emersion heater, but now that has stoped working and it sounds like its boiling dry. Now it is making noises like there is air in the system (similar to what a radiator makes when it needs bleeding, or a washing machine filling with water). The noises have got louder during the day, so I switched the boiler off at the mains. The noises stops, but I could here air escaping from somewhere for a brief while afterwards.

Can anyone help? THere is a cold snap supposedly coming this weekend and we have guests visiting and our plumber won't come till next week..