Does this need to be completely re-roofed?

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Hi everyone,

So for quite a long time this roof over the bathroom/sheds at the back of my mum's house has been leaking pretty heavily every time it rains. However, there never seems to be any leaks on the half over the bathroom which is why she hasn't sorted it yet. From the pictures, you can see the roof is in poor condition with a lot of cracked slates, the lead flashing looks worn out and isn't laying tight against the slates, and a few of the rafters and sarking boards look rotten from the water damage. Between the boards and slates, I think I can see F1 felt which I imagine is also damaged. I can't see the underside of the roof from the bathroom side because there isn't any access into the small loft space above the bathroom ceiling but my guess is it'd be the same.


To the left of the ridge there are two sheds and on the right is the bathroom




Shed side




Bathroom side




Inside shed - where roof meets wall between two sheds




Inside shed - wall with bathroom




Rotten rafter in shed

So my main questions are:

• Do you reckon this roof needs to be entirely replaced with new tiles, membrane and timber? From what I can tell the roof would need to be taken off anyway to replace the rotten rafter so when putting it back together would it be best to replace everything or reuse what is still in okay condition?
  
  
• Is this completely outside the scope for DIY? I want to attempt it myself given that it is a small roof and I enjoy DIY but maybe I'm being way too optimistic seeing as I have no experience. Also from a bit of reading, I think it would require building regs approval if the roof needs to be entirely replaced which puts me off as I know nothing about this process.

If it were feasible to go the DIY route my plan (based purely on youtube videos) at the moment is:

1. Remove all tiles, battens, felt, and sarking boards to replace any rafters that look damaged.
2. Put either the same or new the sarking boards back on. I've read that they aren't commonly used south of Scotland but also that they can be important for roof strength so I assume that's the reason why they're necessary in this case.
3. Add a new membrane over the top of the boards, nailing into the rafters and lapping over the ridge. I'm unsure whether it's best to use felt or a type of breathable/non-breathable membrane.
4. Add counter battens and battens at intervals depending on the tiles used.
5. Add tiles.
6. Add flashing.

Anyways, I appreciate any advice!