Mark, we are not getting water like you which seems to be quite a lot. You might see if you can get a good look from above and make any little change there. This whole window and guard is a mildew problem for sure and I can't see there ever being enough ventilation without leaving it open quite a lot. The mildew needs to be cleaned all around the edge and a tiny bit of diluted bleach probably would not hurt. But yours sounds as if it has a problem on the top or sides when closed, maybe not bent quite right or tight enough.
Cathy. Floating Cloud
"Live in the sunshine, swim the sea, drink the wild air.... "