Help a novice - what's going on with this roof?

Hi everyone. DIY novice here, looking at buying a late Victorian terrace (SW London) that we know will need quite a bit of work so will be getting full survey and trades in for rewire, window restoration etc.

Would be really helpful to get your thoughts on the roof (large end of terrace, hip and gable roof with dormers forming part of original attic storey). We already know that the slates will at least need some repair if not a complete replacement as they're original and not had much attention. My concern is whether we need to do anything with the timbers underneath too. There have been some leaks through the slates over the years - sellers say they were patched up a few years ago and everything is now dry. Though the damage they created has not been fixed, e.g. some parts of the lath and plaster ceiling are visible in the original attic storey where the leak damaged the plaster.

Looking into the eaves I see a lot of very dark brown / black wood (sarking boards?) - see pic. Any chance this is some form of treatment of the wood? Or does it look to you like the wood is mouldy/rotten? I'm wondering which trade we would ask to look at this, and whether it seems we will need to repair or replace roof timbers in addition to any slates. Really appreciate any thoughts, thanks.

I see a damaged Lathe & plaster Ceiling.
And, what appears to be a cupboard that has been boarded and painted on the underside of the rafters.. Not quite sarking
Just based on the pictures posted

Thanks datarebal. Yes, I am fine with having to fix the lath and plaster ceiling - the damage was caused by a leak in the roof above which has since been patched up. I'm more worried that the view from the cupboard into the eaves shows a lot of dark coloured wood. I couldn't work out whether this wood has been painted/treated or if the black colour is actually a sign of mould or rotting.

I assume that replacing the tiles on a 100+ year old roof is a good idea, but that having to replace the timbers would be a bigger deal and more expensive.

I'm not sure you are looking at original timbers.
It looks like the cupboard has been boarded up and painted.
Rafters are hidden by the black painted board.likley you could ease a piece off and look at the proper timbers

Before you start patching up the lath & plaster ceiling you need to look at insulating that area.

Thanks!

Understand we should insulate and check ventilation etc as part of repairing ceiling and retiling roof. Was just trying work out if anything else we need to think about / cost in.