How can i fix my halstead ace 24 boiler?

My boiler keeps flashing fault code 3, when this happens we have no hot water or central heating working. However, after many attempts of trying to get the boiler on it works again. We then continue to use hot water but then the fault code 3 will then come back on. This has been happening every day for the last few months. We have contacted our landlord but he thinks its low gas pressure and we need to ring the gas board, is this right?

Any advice would be greatly appreciated.