Question for British list members

[Carol]

As promised, I have a question on British usage (with more to come,
I'm sure) relating to my latest editing project. One chapter concerns
the collapse of the World Trade Center. (I've already consulted with
the staff of the Chicago Manual of Style on that one--you can't change
the name of a building to conform to British spelling, so it's
"Center," not "Centre"). 

I need to know how the British generally refer to the date on which
the Twin Towers collapsed, September 11, 2001 (or 11 September 2001
typed British-style). In the U.S., it's referred to as "September
11th" or, more commonly, "9/11." I noticed that my author is confused
by her own usage of "9/11" and keeps speaking of the events as
occurring on "9 September." (At least, she doesn't think that they
occurred on *November* 9!) 

So, Geoff or anyone else who can help me: If you're talking about the
Twin Towers and referencing only the date to a fellow Brit, how would
you express it? "9/11"? "11/9"? "11 September"?

Thanks much!

Carol, who wants to be correct without being confusing the author's
prospecive (British) readers