Brash Singh – thank you so very much for taking the trouble to make such a detailed reply and to provide the code suggestions.
Despite my feeling I am not skilled in this area, I was able to understand all you supplied – it was very clearly explained.
Although your solution (1) is more of a hack, it does have the advantage of, hopefully, surviving BuddyPress updates, so I tried that and it worked! It is looking good: http://ovni-owners.org.uk/members/antipole/
It did break some other custom filtering of profile dada that I had already implements, but I have been able to fix that up.
I doubt that I will be alone in wanting this. I think it would be a good BuddyPress enhancement to include this as standard, or, at least, provide a switch in the settings panel. Can you feed that into the development channels?
thanks once again for your help.