Fireboarding RSJ at floor level

My wonderful builder left me with this mess after not quite finishing my extension. This is the bathroom above the kitchen where an RSJ ended up encroaching into the room at floor level.

I want to tackle the puzzle of fireboarding it myself, but am not quite sure on what needs to be done to comply with regs.

Questions are:
- can I board down to the tiles, or do I need to cut the tiles down so I can board further down?
- Do I need to use fire board above the RSJ to cover the joists and wallplate, or is regular plasterboard fine here?
- any ideas on how I should finish the corners and make it look ok? I’m going to have the two changes of level to deal with. One above the steel and another where the plasterboard meets the existing wall above.

All advice appreciated!