Hi all,
Well managed to get my rear under floor/diffuser finished. I took notice of what Phil said and with a bit of tinkering I ended up with this. All inlets are now smaller than the outlets.
I've also drilled quite a few 60mm diameter holes to help remove some of the air built up in the rear tub.
As you can see in the next photo some of the air built up in the rear tub can be released above the diffuser in the slot I've created.
I also decided to make angled cover plates for my lower wishbones while I was at it, should now clear on full droop...