Can I knock down a wall which has a pre existing RSJ running above it?

Few questions? Why was the RSJ put in? Was it always a plan to remove the wall? When was the RSJ put in? And, how much of the wall will you be taking out? I’d get a structural engineer to take a look. As long as the existing RSJ meets structural standards then it should be fine to remove the wall.

In these instances you could assume that due to the fact there is a RSJ, the wall in question is therefore not load-bearing. Meaning you should be all good to go. Another tell tale sign of this is that the brick/blockwork will not be joined to other adjacent walls. (An infill) However a little caution never hurt, i would personally always go for supporting the RSJ during demo, most likely in the form of Acro supports. At this point due to the the fact i cannot rely on an assumption alone due to liabilities, I would advise seek a second opinion from a structural engineer. If there is a hidden detail, the engineer will want you to erect pillars at each end of the RSJ to support the steel.