Sorry for the delay in getting back to this thread. All seems to be working today just as it should. vanadium's suggestion of toggling the setting "Tweaks | Top Bar | Application Menu" was all that was needed.

Testing against a live session of yesterday's build of 18.10 I previously saw many more prompts to restart my session than I when running the live session. These prompts no longer appear when I open Tweaks but only when I toggle the Application Menu setting. I don't know whether there is a bug here, a clash with an extension or whether it was down to a simple human error.

Anyway, I'll mark this as solved on the basis that the simple toggling a setting was the correct solution even though for some unexplained reason it didn't work first time.

Thanks to all that replied.