Advice needed for DIY repairs

[optimystery]

I have an area of wall below a parapet on a 1900-ish house, which in the past suffered severe water damage.
I have had a rubber flat roof installed, and the parapet is in better condition, although the roofers neglected to solve a problem which may be leading to further penetrating damp.
I've attached some pictures including a surveyor's suggestion. This is a second storey so will require scaffolding or a cherrypicker. Basically the soldier course looks in very bad condition and I think could be letting in water during heavy storms.
Internally it has looked terrible for a long time, and for a variety of reasons, including wanting to be sure the roof fixes had solved all the problems, I left off redecorating.

Now I am planning to get the vulnerable section above the soldier course, below the (twice weathered coping stones - they made a mistake there!) filled in and covered over as per the surveyor's recommendation.
Once that us done I am wondering how best to deal with the internal wall. There looks to be quite a bit of efflorescence.

I plan to hire an internal scaffold platform and deal with this myself, hacking off the plaster back to the brick, raking out and repointing with NHL3.5, and then either attempting to lime plaster myself or getting someone in to do that.
I had considered a breathable tanking slurry to block any efflorescence, but not sure if this is the way to go.

How would you tackle a problem like this?

 

[ThornhillMike]

Having just had our roof stripped and relaid and the original (copper) box gutters renewed all I can add is that I went for the more expensive option of having the covering you suggest done in dressed lead. I figured that anything else would have an earlier lifespan that would force its renewal in my lifetime. The benefits in terms of internal drying out have been quite surprising and so far looking really promising...

 

[malcolm]

A lead top hat would be a good idea. An alternative to an infill would be screwing a bit of wood to the edge of the parapet and putting the lead on that so it drips past the lower part of the wall. The drip edge is the right thinking. Taking the lead all the way down might leave it exposed to the wind but a bit of wood below the flaunching might be used to secure a stainless strip that the lead could be knocked around. If using wood might be better not doing an infill so the wood can dry out.

 

[optimystery]

Thanks, good to have the 'top hat' theory confirmed, and why not use lead if I'm going to pay through the nose for scaffolding.

How about the efflorescence on the inside? Anything special need to be done there, or would sealing it on the outside mean that simply re-plastering would be sufficient?
The internal repointing will likely be turning to dust, so presumably that would need to be done too..