Was an RSJ Needed?

We have lived in our house for 26 years. The previous owners had the wall removed between the staircase and the living room. As far as we can tell there is no rsj supporting the ceiling. We assumed this was ok as we thought it would have fallen in by now 26+ years later. We have now noticed that the living room ceiling is now bowing slightly although it is only noticeable when standing on the stairs looking across the celing.

I'd welcome any suggestions as to whether we need to do anything at this stage and if so what can we do?

I would have thought that the first thing you need to do is get a structural engineer or other such expert out to survey your house. They will be able to clarify the whys and wherefors of your own individual situation.