Discussion boards currently only allow 1000 posts per thread and a few I've looked at recently see active conversations end when the last allowable post is made - the conversation have (as far as I can see) then ended. The "Start a new thread" option is there, but hasn't been used. I propose introduction of a feature to automatically start continuation posts.
Some savy / proactive users have worked round this, by opening a new thread early enough and posting a link near the end of the full thread, or the OP can amend their first post to link to the conversation continuation.
So the proposal is that mumsnet only allow 999 response posts on a subject, the 1000th is a special reserved post for mumsnet which automatically opens a new thread (same title and OP post or link back to prior thread) in the 1000th post on the old thread a link is provided to the continuation discussion.