Screed mix to damp proof a concrete clay Subfloor which has a suspended timber floor


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Hi All,

I'm in desperate need of your help and advice on a damp issue I need to resolve. I have had various diagnosis by surveyor specialists confirming lateral and rising damp (I'm not going to debate the whole it's a myth concept so if you don't believe it exists then please spare me and yourself by not replying).
It's due to my house, a semi, on a down hill slop, is attached to my neighbours house which is on the uphill side, meaning they are on higher ground. Their ground and DPC is about 1 metre higher above ground than mine.

This means that their foundations, mainly clay soil are butting against the party walls on my living room walls and chimney walls. This means everytime we get rains, their ground water is seeping into my living room walls and soaking them. I've hacked off the plaster months back and have had the brick walls bare and dried out so I've literally been witnessing the bare bricks get soaked following rainy days and dry out during dry days. I then decided to open the timber floor to clear the subfloor which was also full of concrete and earth type screed which was heaped/lumped against the party wall subfloor footings. Turns out this was done to seal the ground water seeping from lower party walls and also from under the subfloor because the moment I removed this, water started seeping through the loose mortar areas of the lower party wall as well as rising from the subfloor ground and pooling where i had removed the heaped earth.

So to remediate this, I can't really build drainage to take away this water as the outside is slightly higher ground ( I have dug out drain trenches around the whole house to lower ground and let the bricks breath) This has reduced /stopped the rising damp so the only issue remaining is this lateral damp from next doors raised ground against my party wall. So the only option I'm now going for is tanking/rendering the party walls.

1. I need block/seal the seeping water from my subfloor mortar first (scrape about 20mm or more reporter with water proof mortar)
2. I need to also create a concrete screed to stop the ground water rising and pooling, so it spreads through the clay downhill rather than up to my subfloor. The subfloor has some cement and some clay areas. The water is pooling in the clay areas areas that don't have cement (it was cracked and I've dug it out).

Questions
1 So what's the best mix for the waterproof mortar to seal out the seeping water from the few mortar areas of the lower party wall subfloor? Sbr ratio to cement? or Sika slurry etc? I know sealing the water rather than draining, will mean it will rise on the wall hence why I'll tank the party walls. I have drilled a cream DPC (Dryzone) which I'm not convinced works TBH.
2. What's the best screed mix for the subfloor ground which will bond to clay and cement and soil areas to seal the groundwater from rising and pooling?
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