Thank you for your reply.
You can make more space available when you delete texts and attachments—like photos, videos, and files—that you don't need anymore.
That's the part in Apple's misleading statements which is the most irritating. It is giving a false impression. Using Messages in iPhone or on Mac will simultaneously store all those messages & attachments in iCloud (if so chosen in one's Settings, say if you want to access the same messages on different devices), taking up enormous space over time.
But unlike Photos or maybe other files it seems you CANNOT thin the herd within Messages, in iCloud. It just won't let you selectively remove things. You can indeed select individual messages or attachments LOCALLY, on your iPhone, and probably free up space ON YOUR iPHONE. But iCloud storage will not reflect this editing process. It will still keep all your bloated files saved in the cloud, NOT allowing you to "make more space available when you delete texts and attachments." So Apple's misleading assurance that you can edit things out in a straightforward manner is simply false. My iCloud storage space of 5GB is apparently still all used up.
It seems you can only delete ALL messages, the whole lot, including all attachments. Then everything will be gone. This does seem to free up space in iCloud.
That's the way it looks to me at the moment.
If I'm missing something, or if there's a better way I would welcome any pointers.
Thanks for your assistance.