Onoff

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...

Roman snails do you? We have loads here. Migrated or whatever they do from Lullingstone or something. The big ones are near tennis ball sized.

You could eat the nettles tbh. Really good lightly boiled with proper butter and pepper.

Like walk on glazing?

Spit on it?

Overlapping lead flashing joint?

Worth checking the little wires that connect to the sensors, they can corrode. I dip my mine in silicone grease or ACF-50 and slip a bootlace ferrule on if it'll fit. I then dip that and screw into the photocell terminal. Lasts indefinitely.

Told ya! https://www.lbc.co.uk/world-news/kill-switches-solar-panels-sold-china-sparking-fears-energy-grid-attack/