Thank you !
That was the issue - but now I have a second obstacle !
The language in the status bar shows English (United States).
When I click on that I can choose English (United Kingdom) and I click on Set as Default and then OK.
However this does not work - the next document I open has English (United States) in the status bar.
I've restarted Word, in case that was necessary - no difference.
Any further ideas please ?

PS1 In Options . . . Language . . . Office authoring languages and proofing . . . "Manage languages used for creating and editing documents, including proofing tools such as spelling and grammar check" . . . English (United Kingdom) is set as the preferred option.

PS2 The individual styles used on the document are all set to English (United Kingdom) but pretty much whatever I type or do, sooner or later the language reverts to English (United States) I've even tried removing English (United States) as a proofing language - but that doesn't work either.