Thanks guys.... It sure has scrubbed up well.
I cleaned out the plenum yesterday.... I've never seen so many leaves and crap in a plenum before and not a spot of rust!
It does however let a tiny bit of water in the left side... it's coming from up high but because of all the aircon stuff behind the glovebox I can't see where. Drain is clear so I guess it might be the air flap not sealing properly.