Dodgy previous renovation - best way to remedy

Hi,
Looking some advice for best route to take with my Victorian semi. The previous owners removed upstairs and downstairs chimney breasts (including stack in roof) but then just really bodged rendering over the area to the point where the render was held on the walls by the thick layers of wallpaper over it and there seemed to be some drafts and damp around the ceiling/walls joins. In short I've taken the walls back to brick and in the process discovered the plasterbarod ceiling is not supported by anything at its ends and has large gaps which I can see up into the roof through around the edges and some of the joints are not over joists away from the walls.
I didn't take any pictures before but have attached some of afterwards. my question is what do I do about the terrible state of the area where the chimney breast was removed. I was wondering if I could repair the pointing and clean them up then attempt to get some level plasterboard over the entire wall, but wasn't sure how hard it would be to make the plasterboard level which such a recess in the wall. I also wasn't sure if hygroscopic salts could still cause me an issue if I frame the wall and attach the plasterboard to battens rather than dot and dab (which I would need to gets someone in to do as I've never done it). Or do I just get someone in to render over the lot then skim coat with plaster?

As for the ceiling - as the wall is unlikely to be as thick as the huge amount of render I removed and there was already a large gap between plasterboard and ceiling I'm assuming it might be better in the long term to just take down the lot and redo it with proper support for the sheets of plasterboard? Or does anyone have any ideas for retrospective framing/ways to support the ends. I haven't pulled up the insulation in the roof to look from that side yet but happy to if anyone thinks it will help me.

Plasterboard it

I don't know enough about the internals of chimney stacks, but I would be tempted to ask next door if they have a chimney in there building in the same location to put a little smoke bomb in there fire place to ensure nothing comes through.

Also, if they have removed there stack and this is essentially a single 4" brick ( as opposed to the 9" ) acting as the party wall, I would be tempted to sand and cement it up then plasterboard over to reduce sound transfer.

Good advice Paul, then strap, insulate, vapour barrier, plasterboard.

Thanks for the advice - This would be the rear chimney stack in my house and my neighbour has a loft conversion and there is only the front chimney stack in there, so I can only assume the whole lot has been removed on the other side of the party wall as well. There is also no chimney stack to the rear of the roof, so very possible there is only one brick!

I would like to improve the sound proofing but I'm a bit worried about creating damp issues with insulation as it's all original lime mortar pointing. My understanding of how you create damp with inappropriate insulation is really not great so if anyone has advice/tell me I'm being over cautious would be appreciated.

Alastairreid said:

Good advice Paul, then strap, insulate, vapour barrier, plasterboard.

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pardon my ignorance but what do you mean by strap?

MyOldHouse said:

I'm a bit worried about creating damp issues

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You won't. Use a load of rockwool

MyOldHouse said:

pardon my ignorance but what do you mean by strap?

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Build a partition or batten/strap out directly to the wall.

Alastairreid said:

Build a partition or batten/strap out directly to the wall.

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Thanks for clarifying, I was going to batten directly to the wall after I sort out some of the pointing