All,
A colleague has created the following legend in ArcPro from a polygon dataset using unique values, so this is not a raster colour ramp. We simply turned off border display and change a legend property to reduce the gaps between the patches to zero.

But the client wants the symbology to be horizontal, here is a crude cut example by me using MS paint of what they want:

I can't work out for the life of me how one rotates the items in the legend to be horizontal, any carto experts out there with a killer solution?