Just to add some extra information.
Yesterday I removed the front wipers and scuttle tray to have a look for potential leak points.
there was definitely water getting in through the pollen filter casing as there was a puddle in the bottom of the fan housing. Dried this out and removed the foam seal around the filter housing and replaced it with Butyl.
Next up was to remove the puddle of water on the drivers side, a quick clean up of the draining channel and a bit of silicone around the boost gauge hose (just to make sure) I was happy.
Back on the passenger side I starting cleaning out the drainage channel, was surprized to find it totally blocked up with twigs, dirt and leaves as well as an M12 bolt that had got wedged in the drainage channel!!
This was obviously acting like a plug in a bath, which has caused the seal on the filter housing to fail through rotting, the water has then drained away by the easiest path . . . .down into the fan housing.