Ant and Kieth shush a minute this bloke still got wet carpet... Took me months to sus this. If all else fails there is a screw that goes through the plastic facia directly above the pollen filter cover, which if not sealed will leak rain water directly into the duct by the bucket full. I have pics and even a bloody video of water pouring in here but on my other pc. So check and overseal with silicone sealant.
Hopefully that should have sorted it but will test it with a hose tomorrow. Thank you.