Restarted on the build - any views on how to waterproof the cavity/retaining wall like a basement?

[AdTee]

Hi all, 

Some of you may remember my posts from late last year where I had to pause my extension to get an SE to detail how to prevent my neighbour's conservatory from sliding into my extension... https://forum.buildhub.org.uk/topic/40498-i-am-soooooo-stuck/

It's been a tough 8 months with 2 sewage leaks and a water leak all beneath the neighbour's conservatory pouring 'fluids' all over my foundations (not helped by their lack of build-over agreement with the water co.).Since then, I've been researching how to waterproof the lower part of the wall (blockwork cavity wall with concrete infill), and think I'll definitely be using a bitumen self-adheseive layer over a primer. BCO suggested Bituthene, but Permaseal looks good too and is slightly cheaper.

Question is, as it's only a 285mm gap between the two buildings, direct rainfall should be minimal, so do I need to worry about hydrostatic pressure, or even their vent pipe leaking sewage (as it's the head of a 5-cottage sewer run)? I've considered if there's any way to fit a dimpled drainage sheet between the waterproof layer and the concrete, but that would need something to drain into, and a perf pipe underneath all that concrete would be pointless (unless perhaps I put a layer of EPS between the permaseal and the Geodrain dimpled mat creating a cavity below for the perf pipe?).






Would be great to hear your opinions on this one, please!

Thanks