Thanks for suggestions so far.

One challenge is our organisation is very wary of installing any additional code (as Alex suggested) or hooking up something like VS Code, to the live server, which is passing patient information between the various hospital systems... The test system has a little more flexibility, but organisation is a bit twitchy!! Would a plausible path be to sort of follow Eduard's suggestion, but:

1. Export code from live (an export of the classes via Management portal, or if better an export via Studio) to XML
2. Export code from test (as above) to XML 
3. On a local laptop, import live code to a newly created namespace and commit from there to a GitLab repo
4. On same local laptop, import test code and commit from there to the GitLab repo
5. Compare as suggested....

Shamus, thanks, works perfectly. Much appreciated.

Thanks Ben, good start for a conversation in our team...