Well I didn't expect this !
I contacted Office 365 support and, after much experimentation, they did the actions below, which fixed it.
Windows 11 . . . Settings . . . Time & Language . . . at the top there is a section "Windows display language" which was set to English. The technician added a new language "English United Kingdom" which had to be downloaded, signed out of the PC etc.
On re-starting I was able to set the default proofing language in Word to English (United Kingdom) and this time it has remained set (I hope !).
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