Quick update - I've been trying to eliminate more possible areas of water ingress, and discovered this today. (See pic below) The rubber bung is missing from the right hand (drivers in the UK) front wheel arch inner panel.

The opposite side has the same hole but the bung/stopper is present. This panel is quite close to the footwell area. Water kicked up from the tyre must be getting into this hole. Luckily I had a spare bung of the correct size. Anyone know if this might be connected to the damp carpet in the footwell?