Wet underfloor, wet wall, no leak detected. What next?

Eldi_Dos

How well do you get on with your neighbours, could you ask both next door neighbours to lift their floorboards and see if theirs has any issues.

greenbee

When I was looking at houses a few years ago, an architect friend told me that damp problems at the bottom of a wall are often caused by issues with the gutter at the top, so it's well worth checking that the gutters aren't blocked, leaking, dripping, or getting overwhelmed by some of the torrential downpours we've had recently. If they're not working well that can result in water pouring down the wall and/or pooling at the base of the wall. 

ThisIsWeird

Helpful water guy!
I think I'd be focussing now on external causes too. That essentially comes down to rainwater goods, underground drains/sewers, and the water table. 
For the first, is there a DP coming down anywhere close to that area? If so, could you take a photo of how the DP enters the ground? Is there a fitting you can detach so you can investigate further? Or even to observe that it does drain away quickly to wherever it goes?
What is the ground surface outside this area? Can it be lifted/dug in to easily? If you can do this, and find it 'pools', then that could also be a clue (tho' not sure of what...)
Do you know where the main drains and sewers run - do any travel roughly in that area? Eg, is there a bathroom in line with a manhole cover outside? 

There are seemingly two issues here - one being the seemingly unusually wet ground/found, but the other is how that water is getting to above the DPC. Can you work out the extent of bridging? Is it very localised to that corner? I wonder how easy it would be to jack and support the joists at that point, and slip a new DPC under them? Ditto with the wall - any way to cut into a mortar line, more than half way, and ditto. Repeat from the other side. But, you'd need to know what you are doing (I obviously don't...)