Justine, I am horrified at your response to being reminded of the Mumsnet copyright page:
"We copied our copyright notice off someone else eons ago iirr - possibly Gina Ford - so not sure how efficacious it is tbh and as I've said we're in the process of changing it to sound far less stringent - Gerry's got the new stuff up her sleeve but she's on hols right now - but you're right it is a bit nonsensical because my understanding that it's not really a tenable position wrt to fair use, but as said, I'll check."
You must be conscious that some people have posted intimate stuff on here because they feel reassured that their details are going to stay within Mumsnet and not be used by anyone else for commercial purposes. They have felt this because YOU have told them so, in no uncertain terms. You state that no one but Mumsnet will be allowed to use things posted on here.
If the statement on the copyright page isn't true, Mumsnet has misled its members potentially quite seriously.
Additionally, that you have raised practically no objection to this DM journalist lifting what is essentially a whole thread (ie not just one or two funny unrelated quotes as other papers do), then stating that you have agreed between yourselves that she will not use anything sensitive or which could identify the posters, when the thread in question is clearly both extremely sensitive and can easily identify the OP, is frankly both astonshing and deeply disappointing.