I don't know the make of our boiler and it came with the house - it is at least 12 years old. We have it serviced every year and it has been working fine until yesterday. They day before that, an engineer came around and freed up the valves on two of the radiators by tapping on them. He suggested that, to prevent further problems we put some Sentinel X400 into the system.
I partly drained the system- enough to empty the header tank and added the liquid. What happens now is that the boiler does fire, but not for very long and not very often. We can hear "gurguling" noises when it does fire. Is there air in the system and do I need to tamper with the pressure relief valve?