As above, once these are fitted in place is there meant to be a bead of silicone ran along side the inside where the frame meets the tray?
Also in this bathroom when you run the tap, the water runs away and a few seconds later you hear it running in some pipework, is that normal as never heard that in other bathrooms or is it perhaps where the pipes are running.
Rust isn't anything I have ever seen on a shower screen. Are you sure it is not mould growing? That can sometimes be brown in colour. Will it clean off?
As for "There is some silicone on the posts either end both running vertical and horizontal" - as I say, all vertical joints inside and out should be sealed, but the tray should only be siliconed from outside. This is simply to prevent water from becoming trapped in the framework of the cubicle. Leaving the gap open allows the water to drain away into the tray.
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