Okay so update. Turns out it's not from the front at all! Unfortunately, this was found out due to the recent storm bringing more water into the car.
The front is drier than the back, and the air filter is bone dry so it's not coming in there.
It appears there's a leak into the rim of the spare wheel compartment, and when the car is leaning left on a cambered road (as it is outside my house), water can seep over this ridge down past the rear passenger seat and into the carpets at the front. Whereas ordinarily it would drain into the spare wheel compartment and then through to the floor.
Why there's water getting into there I do not yet know, that's a job for tomorrow / the weekend!
It appears to be entering from below where the small black clip is (possibly for baby seats?) Just behind the rear passenger seat. Also the fabric trim there is soaked.