Load bearing or not?

hi i have a 1970s extension on the side of an old 1880 welsh cottage. I have a question about the 1970`s part. I want to take a wall out from down stairs there is no walls above. Its a concrete block wall that runs up to the joists but in between the joists and the block wall is plasterboard. If this was a load bearing wall why would they put plasterboard in between? The plasterboard runs all the way through, there is no way it could have been fitted after the wall was errected. The joists are 47x170 and 420 apart with out the wall the joist would span 5m. The joist are not on hangers. Iv looked at the tables and i can see that the joist would not be strong enough without the wall, i think. Is there a difference in standards to 1970 and now? But why the plasterboard?