Onoff

+1 😃 Though I do know a drawist type who knows their ways and has a successful track record of dealing with them successfully.

What about the other sides of this concrete column? Get in there with a thin vacuum nozzle. Get it as clean as then flood with the black stuff? Difficult to see without a bigger picture but I appreciate you don't want the BAU to know where you are...

So essentially permeable? Just sloshing blackjack esque stuff over the join between that and something more solid, I wouldn't get my hopes up.

Was is this "rough ground" made of?

A rough substrate is no issue. More so if it's loose and bitty.

It says for professional use. You need to take it back.

The liquid rubber will stick to both substrates and hopefully allow a flexible joint. The surfaces want to be clean as in vacuum job I would and maybe a wash / dry beforehand.

So you have proper, traditional, hot laid asphalt? Anything else is not asphalt. Atop that a membrane then sand bedded pavers? Looks like green mineral roofing felt in earlier pics with all the nasty associated joints, laps, folds and seams. Pay particular attention where it laps up any walls etc. Hot laid asphalt is great and forms a homogenous layer even on upstands where you incorporate fillets. Until of course it cracks. I'd assume the flat areas are ok and focus on the bold highlighted bits.

Make sure you ram it into any deep / tight cracks.

Um... https://www.amazon.co.uk/Resincoat-Waterproof-Technology-Long-term-Performance/dp/B096BK1127?

Just paint the lot with a flexible bituminous paint, concrete, felt, overlaps etc. You've too many maybe points so "tank" the lot I would.

Takes him back to his days cleaning hot tubs!

Years ago you could get something similar to seal around windscreen rubbers, always a worrying ingress points on old Fords! It was called Seek 'n Seal. This is it's modern incarnation. What have you got to lose by going over EVERY visible seam with it? https://www.grime-x.co.uk/product/grime-x-seek-n-seal-windscreen-sealer-sealant-liquid-rubber-silicone-repair/178

Plenty of this sort of stuff on Temu:

I forgot to put the tongue in cheek emoji as was only half serious but tbh, why not? If the dwelling was "airtight" then wouldn't a leak path also allow air to escape? Thus you ramp up the pressure in the basement sealing everything else up then go around spraying some leak detection fluid on suspect seams. It forms a thin film & any air leakage bubbles it up. https://www.tyrebaydirect.com/product/leak-detection-fluid/

How about pressuring the basement then going round the roof with leak detection spray?

Guaranteed even if he doesn't fix the leak.

"Ja, ja, das ist gut! 😉

I reckons the "solution" is something like this...

How about putting a walk on, glazed roof over the whole lot?

No good for @Pocster he doesn't buy cheap!

Myself, via a wander lead, then to ground as part of my earthing therapy regime...

Speaker wire? I've always used twin and earth.

My first thought that.

A layer of broken glass used to be put down around here before they concreted the slabs...