To Nick:
Remove the Fusebox cover and check for water around there, if there is, open the bonnet and on the drivers side under the plastic flimsy scuttle tray you should see a bonnet cable or some sort of wiring going inside the cabin through a rubber grommet. If this is cracked or perished, your water is coming in through here. Any leaves or muck under the scuttle tray will block water drainage through the wing and the water will go in the car if there is no other way out! Try and clean the muck up.
I found "Instant Gasket" / "Silicone Sealant" good to put around grommets for Sub wiring or perished grommets as its water proof.
Generally the culprit is the plastic door membrane behind the door panels being perished, get the door card off and make sure its secure. The window seals lets water go past and fills up inside the door if the door-drains are blocked. The only thing preventing the water going into the car is this plastic sheeting. If you see rips/tears/or its not secure around the edges fix it and see how it goes.
On mine, after a lot of pssing around, I found the sunroof drain had come apart from the A-pillar and water came down from the headlining. Never saw it happen when i was driving but when i parked up and left it you could see it dripping.
Another passage for water into the passenger foot well is through the heater fan. This could be because the scuttle tray isn't secure or has some cracks. Directly underneath is the pollen filter mesh going to the fan. New scuttle trim or gaffa the cracks up.
