Bit of builders advice please

This is the old side of the house (build 1927). The wall is single skin with (now) a 50mm dry lining inside.
The pic shows a drain and the other side of the wall is a fireplace in a room about 8m x 4m. I can't explain why but the floor is about a third floorboard with air under and the rest seems to be solid concrete slab, including the area with the hearth.
Recently we had heavy rain and it started to smell damp. The hearth became mouldy and i was clear there was a water issue hence the digging!
It's specific to this location and the water in heavy rain floods off the roof as it flows around the chimney above (which is covered and not letting in water - already checked) and creates a waterfall by completely missing the downpipe.
You can see the soil is too close to the dampcourse so that's getting reduced but I'm curious about that sealed gully. It's looks to be storing water (absorbing it and storing) but I have no idea why it's even there. Before I smack the concrete lid open, does anyone have any bright ideas on what I might do to remediate?
I'm currently planning on several things- reduce the soil level, add hardcore around the chimney / flood area and to repoint a few areas where the mortar is failing. Also pondering removing the brick perimeter to the gully as it's built up to the dampcourse.
But I'm most interested in that weird gully!

 

I know very little about building so I may just be confusing the matter.

Is the area below the sealed gully solid or is that soil I can see? It reminds me of what could have been the bottom of a stack pipe for aerating gullies or drains.

 

Couple of extra pics if it helps but the main bit is why is there a filled in gully?

Air bricks are fine and for the area which is a timber floor with air underneath. Yes, as mentioned the soil is to the damp course so too high and going to be reduced.

Gully is below the dampcourse but the perimeter enclosure is built right up to the dampcourse which is why I thought about removing it.
Could easily be a soakaway somewhere but haven't seen it backed up yet. Will keep an eye out.

It's the concrete stack Shiney; the bit the drain sits in. It looks as if there was one, then they filled it in and built another one adjacent. Hadn't thought of an aerating gizmo but there's no vent.

Think I'm going to smash it open tomorrow Am pretty sure its just a big load of water soaking through into the concrete as everything is too close to the dampcourse and the water cascading there in heavy downpours is making it worse.

 

Nope! Good thoughts though. I haven't got a pic but the floor is a total shambles, it's like they were using up wood, patching areas and then decided to start with concrete!

 

Thanks Ricky, that's a really good point about algae. I'm going to bust it open tomorrow to see what's happening in there but also have another poke about as far underneath as possible.
Water runs away and it's quite sandy soil so it drains well come what may but it's getting in somehow

 

Maybe there was a sink with the waste going to that second gully.

The detailing on the chimney is wrong, looks like someone has used tiles instead of lead flashing, it's bound to leak.

 

Definitely not coming in through the top of the chimney and can't see any evidence of leakage at the roofline via the loft.
Would certainly explain it though.
Have to go out with the kids for a bit but hoping to look into it more this avo

 

I haven't removed that concrete plug yet as when I was repointing and poking about I found quite a few little areas that had small holes or pockets in them, mostly hidden from first glance.
The area in front of the chimney is now dug out with hardcore under the damp course so not going to cause further damp issue; will get to the rest when it's cooler!!

 

Top tip on the milk bottle! The water gully for the exisiting downpipe looks to exactly as it looks, simple gully connected to drain running off somewhere and doesn't appear blocked. I did find one notable hole just next to it and definitely would be contributing to the problem, now sealed plus has air in front.

 

I have got to breaking open the gully...


As you can see, there's a runoff heading under the house. Never seen that before but it can't be good as there isn't another end that I've ever seen so presumably any water entering is going into a concrete floor...
There's also that whopping hole that definitely breaches the DPC and will be filled shortly. My question is how to best address the drain? Cover and let it dry? Fill with concrete? Expanding foam?