Thanks for the Suggestions.
Well Guys, it looks like you were right.
After your experience I asked myself "what has changed?". Well you were right the previous owner had fitted a Rancho electrically adjustable shock system and a switch for fog lights. Well the pneumatics pipes and wires they'd pushed through a grommet in the engine bay into the driver's foot well.
But because a relay block was obscuring the grommet it wasn't possible to see a grap in the grommet you could push a pencil thru (i could actually see daylight thru the hole). After removing the relay block and getting to the grommet I've sealled the hole in the grommet with silicon sealant from both sides of the firewall.
It appears that rain water was running down the windscreen into the engine bay, encountering the pneumatic pipes and following them into the foot well. I've removed the seat and pulled back the carpet to allow the footwell to dry during the expected hot weekend. I've also thrown a few jugs of water onto the windscreen and it looks like none is now getting into the footwell, so fingers crossed.
Thanks for the help.