i take it black ash render is bad? will be if i got it suppose. Does it need removing internally or ok if water problem solved?
I would only remove hollow parts if you have any. I would remove as much of the gypsum skim I could.
For the outside wall I would lime render with NHL 5. Coastal area have problems with salt that creates extra problems that need thinking through.Cement based renders are no good if your house is lime constructed. Movement causes it to crack allowing water in.
The render needs to go on when it's warm and dry weather. NHL lime with stiffen after a few hours but will take 6/8 weeks to cure. If you can get a few dry days it would be ok but the more dry days the better until it's cured.
You then need to find a good micro porous paint for the lime wall, it will protect the lime render from the salt air. Paint regular.
Clean off brick below bell cast bead and re-point replacing any degraded brick.
NHL 5 is good for weather exposed walls.