Bit of an update on this, I think it is down to the version of the file being used.
If you have the EN-US folder in system32 you are using the English US ISO for your image. If you have EN-GB you are using the English International (UK) ISO. It seems the credprovhost.dll.mui files are not interchangable.
The previous IT guys at one of my schools had been using the EN-US version of the W10. If I took the CredProvHost.dll.mui file from this, edited in ResHacker and applied to the EN-GB version of the ISO I would get blank/no wording at the login screen, If I applied the same file to EN-US version, it worked fine. As soon as I took a copy of the file from the EN-GB version and ran through the same process with ResHacker the custom wording appeared - in short probably worth re-editing the file from a vanilla ISO of your choosing whenever a new build of Windows is released and ensuring you match the right language version (EN-US or EN-GB).
Thanks