The texture is meant to be a horizontal slate brick pattern. I have imported it from Quixel Megascans so it automatically set up all the nodes with the Normal Map, Roughness, AO and Albedo but for some reason it is ridiculously stretched, this is despite me UV unwrapping and rotating the UV Map to line up with how I want it.
I have looked at other threads related to this topic, people saying stuff about sub-surface modifiers etc. but there are no un-applied modifiers (there is very little geometry as you can tell because it is a box!), scale, rotation and all other forms of transform have been applied.
To UV-Unwrap I marked the seams I needed, then when unwrapping it I used 'Conformal' because using 'Angle Based' made the mesh really wonky (the edges weren't at 90 degrees/perpendicular, some lengths were also incorrect proportions - it just really messed up the net of the shape) for some unknown reason - so if someone could explain that as well that would help a lot.
I don't understand why the texture is displaying so bad in the viewport, please could someone help?