Help WTF is this?

[LoveGigi]

This is the state of my bricks immediately behind my front door in the void under my floor boards. Recently my pine floor boards have been sweating over night and we have slugs. The problem has spread to my living room too which is not too far from this wall. There is clearly an issue possibly coming from beneath ground level below my front door step. What are the next steps I should take and is it worth speaking to my insurance company?

First time I've come across @PigletJohn but man, am I impressed! 💪🏻

@friskybivalves

I'm only coming on to say that even before piglet John mentioned the fern I was going to point it out because he had mentioned their relevance on an earlier thread this week and even though I have absolutely no problems with damp courses I always absorb a piglet John tip [girlie swot]!

And to compliment OP on her excellent diagram and for knowing where her stop cocks are. Am utterly clueless on that front...

Best find out before you need to know.

I’m loving all the comments here! PigletJohn you are a hero to many a mumsnetter obvs me included! I’ve been privileged to have previously had some ace advice from you before! My experiments are commencing at full throttle this morning. I’ve cleaned out the gully and have attached a pic, the trap appears in tact but what’s happening underneath that I don’t know. I’ve done the vinegar test and there may have been some very mild fizzing, see pic again. The substance is very gritty. I’m trying to get another board up so that I can go subterranean and try to wire brush this stuff off!

Damn it! No gully rummaging for me this morning as the iron grill is stuck fast!

the iron grill lifts out for scooping. There will be an opening in the pot to one side, pointing towards the trap. The water level should be above it.

the patched concrete forming a shaft down to the drain is fairly typical of (futile) attempts to repair a broken drain without actually repairing it.

Either the gully has subsided into its lake of mud, or the concrete has been raised so much that it is most likely bridging your DPC.

or both.

ah. might be rusted in, might be blocked by the added concrete

you could try making hooks with coat hangers.

A more vigorous attack might break the clay gulley. There will be time for that later.