Tricky wall conundrum!

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Hi all. My (mostly) DIY timber frame extension build is slowly progressing, with the block and beam floor now in. However, I’m currently stuck on the details of one of the walls and wondered if anyone here has had to deal with a similar situation, or could perhaps offer any suggestions? My architect has yet to come back to me with a solution.
 

It’s an extension to an end-terrace cottage and all new walls will be timber, sheathed in cement board and clad in steel. On one side, the neighbour has a brick-built extension projecting 3m. It's astride the boundary and the party wall will form a part of my build (will become my internal wall). Beyond this, they also have a 4m long conservatory (that sits about 300mm in from the boundary, solid plastic walls to the sides), and my extension will continue parallel to this, and beyond. 
 

At first glance, the tricky parts are:
1. If I build a completely timber wall here, I’d have to clad it before standing it up, and if anything went awry, I’d never be able to get to it as there wouldn’t be room between the two buildings. I'm also concerned about the risk of fire in their conservatory, but that's a conversation for another day!
2. my ground beam and subsequent floor level is below their conservatory floor and footing (looks like pads at the corners but hard to tell) - the ground beneath the conservatory is mostly loose rubble, doesn’t seem particularly solid, and I need to backfill behind my wall to shore it up.
 

Ideally, I’d build this wall in block and bring it up the inside face of the brick party wall (which is where the timber frame was drawn to sit). I’d then build the timber frame inside of this block wall and treat the blockwork as cladding, giving me an outer skin which would hopefully be maintenance-free. This raises a few more issues, however:

3. The first few courses of block would be below ground on the neighbour’s side, adding pressure and moisture (once backfilled) to the wall. I think I’d need to tank it part-way, and add an intermediate pier, as at over 3m high and 4.5m wide a full block wall would be quite slender.
4. I couldn’t tie the blockwork into the frame due to a.) having to build the block wall first and b.) the sheathing, wrap and battens already being on the timber when the frame is erected.

I’ve been looking at the TRADA drawings below, trying to work out if there is a happy medium between the two. It would be great if I didn’t have to build such a high wall but maintenance remains a concern.
 

All suggestions greatly appreciated, thanks.

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It's the wall highlighted in yellow on the plan.