As Cauncy said its a gutter sitting in a rail, that is generally level and runs around a 1/4 of the house perimeter, to 3 drains.
The original problem was that the downpipe fold/cut was done poorly; as in pic B.
Even with nothing "blocking" the gutter, it would still backfill as there was about a 3mm slit for the water to escape through, twice.
This overflowed the gutters into the eaves. I found a single leaf and a couple of roof screws totally blocking it.
(The builder came back to fix.. kinda)

I've added mesh to stop leaves and rocks* getting in.
So theoretically, I have resolved the gutter blocking issue...
Next step would be to replace the downpipe altogether with 90mm PVC.
Which should be easy enough.. I even have the paint to colour match.
Another option would be to put in some pissers.. essentially does the same thing as the slotted gutters. (Just sounds better)


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As said, The hardieflex lining is hard against the window fin at the top. I cant get it to move at all.
The order of how it all went in would probably make it easier to understand.
So fuggit! I'm just going to cut in a vent.
2 Reasons:
1) Airflow; The hardieflex will dry out on its own but there has been mould growing on it, so it stays wet for a while.
2) If the gutter overflows again, it will drain out the vent rather than the windows

*Rocks get dropped by the local crows, smooth round rocks from a house up the road, have no idea what they use them for?