Downgrading DirectX to DX11 is the solve for Windows 11 as far as I can work out, but the dxil.dll file issue that I had originally isn’t a Windows 11 issue as none of the machines I use have Windows 11 installed, both at work and at home.
That’s not to say that downgrading to DX11 on Windows 10 might not also solve the issue I had, it’s just not what I did.