There's something new that's happening which is strange.
The topic that I want to split is a sticky topic. After the failed attempt to split posts from it FB now lists it as the very last post in that board (i.e. as if it was the first post in the board); ignoring the "sticky" bit.
If I unsticky it then it presented as the first thread in that board (which is correct because it also has had the newest reply entered into it).
If I then "sticky" it again it is listed at the top of the board's posts as it should then be (no other sticky topics) *but* the posts that I wanted split (from the end of the thread) now appear as the first posts in that that thread and everything else has been moved down the list.
PS, if I replace the original 4 altered files then all is well again (but I can't split the topic

)