Thank you, braden85, but our problem is specific to France, it seems. We have no problem with our MacBook Pro connecting to any wifi in the UK, either at home or anywhere else. But as soon as we are in France - whether using our neighbour's wifi or any Tourist Information Centre, hotspot, etc, etc - we fail to get a connection. The Mac finds the router, but will not connect to it. And as we have no router of our own in France to experiment with, we can't really start investigating everyone else's router!
However, as we said, our old MacBook connects every time! So we feel it must be something to do with the settings on the MacBook Pro, or the channel frequencies which it is set up to use, or something.
Any more suggestions, please?