So I don't think it is blocked:
https://www.youtube.com/watch?v=FS8w6JqsaJU
It was dry as bone down there until I put a couple of gob fulls of water down there, at which point you can see it all come out the bottom of the car. You're right that plastic tubing is perfect. I actually went down there with an endoscope. All looks pretty clean.
Given I've done all the other drains it must of been the scuttle drains overflowing into the cabin?