Ok, so I’ve copied a file called dxilconv.dll from my System32 folder (C:\Windows\System32) and replaced the file in the folder listed above - keeping the original just in case.
Edit:
and renaming the copied file to simply dxil.dll - as per the original file.
Unreal now appears to boot into the editor.
Can someone let me know if this is an OK thing to do, or will it cause other issues further down the line?