For sure flatnose looks like you're starting this one from the begining,easy enough to save your
tile if you just rip down plasterboard,when you go to put your tiles back up use hardi backer board,or red guard water membrane or you'll be repeating this webpage 5yrs down the road :welcome:
Agreed, I would personally go for either hardibacker or aquapanel screwed to joists.
Remove ALL the plasterboard that is tiled onto and replace those areas with the
tile backing board. Take the time to try to get the walls as level as possible whilst you screw this up, this may mean planing down or packing out joists. It may seem like a lot of work at the time, but you will thank yourself later, trust me:smilewinkgrin:.
Whilst the plaster is removed you'll need to clean up the bath. This will be easier without the plasterboard in the way. I use a new stanley knife blade to separate 90% of the Silicon from the suite, then go over the rest with Silicon eater. I have taken baths out just to clean them up before now.
It would be better to remove the bath and refit it with the new board up, but if you don't then fit the board so it runs about 3 inches below the top of the bath. Pump Silicon around the inteface/gap between the wall and the bath making sure it doesn't just fall through the gap. Get a good seal at this point then do the wet finger trick to get a good joint.
Then
tile, grout etc then apply another bead of Silicon.
If you do remove the bath, before you refit it, run a bead of Silicon around the bath where it meets the wall so that it oozes out when the bath is fitted.