Having exhausted all of the usual suspects-
Roof seam above screen
Fan intake plenum filter cover
Screws holding filter cover, plenum trims and everything else in that area
and still getting a puddle in drivers footwell, but only when the wind or slope where car parked are in a certain direction, I've decided to rip into it and sort once and for all.
This is about as deep as you can get in the leak feed area:
Bit of damp in the screw area at 7 o clock so a definite suspect, but I was planning to pop out the whole plastic inlet bucket pictured and re-seal the gasket and screws. That would mean that any water entering the whole front box under the screen couldn't get anywhere except out through the drain gap at the two ends and would have no path under the plastic bucket into the fan duct.
Trouble is, every screw holding in the plastic bucket is rusted solid and won't hold a screwdriver. Only option will be to drill them out- a major PITA.
In the meantime, as 7 o clock screw is wet, water is still obviously getting in from the top so need to work on that as well.