Seems that the easiest option is to quit Unreal, quit Visual Studio, and then delete the BINARIES folder from the Unreal Project. Then open the Unreal Project again, and say yes to compiling. For whatever reason, that resolves the issue.
I have a second PC in the office and I just installed 5.4 and created an Editor Toolbar Button and got the same result when trying to live recompile. And once again, deleting BINARIES fixed it. Maybe its a bug?
I’m not a C++ programmer so I’m out of my depth. I’m only taking baby steps because I’m trying to write up a spec document, to get quotes from real programmers on making a plugin for me. 