Air vent.

I take it that there's no roof overhang on these gable ends. In which case, you're right to be worried. Those will let the rain in.

You could stick one of these on top of the grille, but it will reduce the airflow:

Or maybe one of those over just a fly screen. There is no need for the grille if using a cowl but you need to stop the critters coming in.

Will it though? The insect mesh should have only have about 40% of its area as open holes, so the open end of the cover only needs to be 40% of the area of the grille to give a full flow of air.

The advantage of the grill is that it gives you an easy way to mount the insect mesh to the wall.

About 50mm/2" overhang, the gable trims (flashings) of the corrugated roof will partially cover the louvres (leaving a gap of say 40mm/1 1/2")...

I'm a bit undecided on this one. It's a corrugated roof, so I wanted to uencourage airflow to prevent condensation. However, if I leave off th foam fillers, which I am very tempted to do, there will be a good deal of air flow from the eaves and ridge. I'm putting a piece of DPM over the ceiling (plasterboard) with insulation over that, anyway.

without foam fillers do you expect any animal visitors/residents?

Doubt it. Maybe the odd wasp's nest, but they are easy to deal with if need be.