Eric, you're a star!
That was exactly it. I first of all made the normal insert page via DataAssist (messaging_Insert.php) but then made a copy of that (called it messaging_Reply.php) added a few modifications and then I got the story already related. What I didn't do was change the Action of the form to "messaging_Reply.php". Soon as I did that, it worked.
I said it was something simple that I just couldn't see. Yet again another pair of eyes does the trick.
Many thanks once again.