God, we've had EXACTLY the same problem as you now for over a year Ry!
On our estate, scuttle drain regularly cleared, seals
seem to be good, though the test is to put a hosepipe on it I suppose. Numerous mechanics had a look and can still not find the cause. :-/
First thing is to get it dry, it stinks when you take the carpet up and the sound absorption material also acts like a well primed sponge! Put an old folded towel underneath between the floor & carpet and try and get it dry. The fact that it is in the passenger footwell means it just finds itself to the lowest part of the car.
Now onto a possible cause. An old(!) member on here and very knowledgable gent suggested it maybe the air conditioning condenser which could potentially give out a cup-full of water at a time if the air-con is working. The condenser is situated just behind the dashboard/glovebox area but alas, I have never actually seen it! Though, it should have a drain hole in the body which exits just above the gearbox on the underside of the car.
This is where it gets bizarre, ours has
supposedly been checked and cleared but still...........wet floor.

Windscreen is possible but ours has still got original windscreen. What's the significance of door locks CB??? All I know is if someone finds a cause, I want to know!