Excellent! Thank you, Maiki!
Well, the default locale for the site is indeed set to English (UK):
However, even if the default locale for the site has been set, the default language for the welcome page seems to be English (US). So when I replaced the English (US) text, then I could see the revised welcome text.
As you suggest, that default language seems to come from my browser (to which I have now added English (GB)).
Does this mean that, depending on the browser language of the visitor, they might see different texts? If so, then maybe I need to add the equivalent text in other languages our members are likely to have as default in the browsers, such as French or German.