Many thanks for all your comments on this issue - at least
I now know that any uncertainties in the value of delta T
are unlikely to be sufficient to push the event back into
night time.

As a postscript, I finally located the source of this idea
via the BBC's web site: there's a book by Dr Michael Molnar
called "The Star of Bethlehem: The Legacy of the Magi". I
emailed Dr Molnar with my query, and his response confirmed
the view that the event is essentially astrological rather
than astronomical - as Robert Orso says, the visibility of
the occultation is not important.

Best wishes

Mike