Tom is correct. The Magic Key will work if you null out the global field on the parent side of the relationship prior to using the Set Field script steps. Or, you can use a portal (including one off-screen).
(Just a bit of forum etiquette, by the way: It's best to mark a question as, well, a question. This encourages people to participate, because they get points for helpful or correct answers.)
Hi Tom and Mike,
Thanks an absolute bundle!
I can't express how delighted I was with this reference. I love Magic Key.
How have I got by without it?
Mike, thanks for the forum tip. point taken.