Managed to strip down the scuttle panel and replace the plenum foam this morning.
Found this on the passenger side:
Didn't get as far as removing the panel above the driver side to see inside the pollen filter housing on the side - I think it needs the wiper mechanism removing.
Cleaned up all the mating faces of scuttle panels, windscreen etc and put them all back. Replaced all the missing screws (several missing along the bottom of the scuttle panel and the A-pillar trims. Filled all the screw holes near the pollen filters with silicon before putting the screws back in and siliconed around the pollen filter covers. It looks like I need to get 4 replacement plastic clips for the lower windscreen trim too, some are missing.
Then I moved inside and stared removing the glovebox to get a better look inside the dash to try and find where the water was coming in.Grabbed a jug of water and started pouring over the windscreen. I couldn't see any new drops coming in so I think it's sorted.
This was about the time heavens opened up again so I couldn't continue with the plan hoover up as much water as I could. Left large bundles of tissue paper covering the carpet weighed down with the rubber floor mat in the mean time.
I hate the glovebox in this car! Tried putting it back in and it wouldn't close. Was in a rush as it was pissing down with rain but managed to get it to a state where I can smack it hard to close. I think I figured out how to adjust the glovebox to adjust the panel gap though, so that's now a new task fore when the rain stops.