Just a quick update for people as I've finally stopped my Leon leaking like a sieve.
As the weekend I removed the wiper arms and scuttle tray to have another look round the pollen filter housing or any other other potential leaking points.
With the housing still in place (filter removed) I flooded the scuttle tray using a hosepipe. It was fairly clear to see where the water was literally pouring into the fan housing. Looking at the front of the housing it was leaking in along the long edge at two points.
I recently 'fixed' the filter housing with a strip of butyl but obviously this didn't fix the issue.
I removed the housing and butyl strip. Just as a bit of a test I replaced the housing and found that it rocked backwards and forwards when in position. After screwing it down I noticed that it lifted the front of the housing up, hence why it was leaking at the front, it was stretching the butyl strip apart and letting water in.
I removed the housing and found that it was actually warped, this must have been through previous attempts to remove the housing. To compensate for the bowed mounting lip I put two layers of butyl around the entire mounting face with an extra layer on the front covering around half of the mounting face.
I put the housing back and scured it in place, stuck the hosepipe in the scuttle and re-flooded it. . . . NO LEAKY!
So, anyone who reseals their pollen filter housing check it isn't warped or you'll be wasting your time by using just 1 layer of Butyl.
Now I just need to get my carpet dry.
Andy