Floor joist damp rot etc.

Had a problem last year, the neighbours downpipe at front of house (on border but on his side) blocking up. It was an old metal pipe. Anyway water was overflowing from hopper on to the wall, creating a perfect flow of water down the wall. Being a 1920s house it has mortar joints. Anyway didn't notice so much until my inside wall in corner of front room got soaked.
Not happy so I spoke to neighbour, it's only a new rain hopper and some pipe, even told him where to find in Wickes. Anyway he dragged his feet, saying it's on his list. I got fed up of waiting and done a bit of silicon on the gaps on wall, he then turned up looking at it, saying usual its on list.. was gonna do it myself. Eventually it got done. Took about 3 months... house is not perfect but never had that much water in corner as its damaged wallpaper this time.
Anyway 6 months on as wall dries out noticed orange dust coming from corner. Assumed must be something from bricks (seems silly assumption after what I found).
Pulled up carpet and ripped off the skirting boards, mostly brittle and weird shape now.
noticed the one floorboard was rotten so after researching this dust, decided to pull up boards. See attached.
Got to find this new source of damp in corner. Sure that drainpipe issue didn't help. And that is a 3rd skin of bricks inside yes to mount the joists? noticed the mycelium on back in video. I was gonna spray it all but source of damp is the driver yeh. Check outside prior drain pipe thinking maybe the water source? Also clean out the old pile of bits there and there was bin in front of one air brick outside.
That just over wall in neighbours old metal pipe with new above it.

You need to make sure all rainwater goes down drain and away.
No cracks around drains or it will erode the ground under.

Those timbers need replacing.

Re brickwork. Seal the outside.


What's against property on ground? Concrete or mud? Could look at aco drainage or french drain around the house..

Thanks. On my side it's slabs. On his side he's got tarmac that eroded at back (from water issue before probably), and did you see the old metal pipe in ground, there's gaps around it. Guess it could be heading down there. Also wondering if the wall in corner on floor is still damp from a few months ago..
Thinking of filling his side up with concrete and sealing wall and checking in a few weeks. Can unscrew my fix.
Was reading about wrapping joists to

Also before I'll started scrapping damp wall (and cutting boards back). Healthy dry rot on that wall, and I think goes to other side in cavity from other video. I was going to saturate from top and spray (I read to drill to, not sure if I need to).
Got this;