scoring emails based on the To and Cc headers

[Mark Longair]

(I originally asked this at unix.stackexchange.com but someone suggested
to me that it would be more sensible to ask in this group.)

I use Gnus v5.13 in GNU Emacs 24.1.1 to read my email, and I'm having
trouble getting a simple score file to work.  Essentially, I want any
mail that's sent to (or copies) a particular email address to be given
a higher score.  My example score file, in ~/Mail/all.SCORE is:

    (("from"
      ("Mark Longair" 1000))
     ("To"
      ("mark-scoretesting@example.org" 1000)))

I'm making sure that that score file is being used by having added the 
following to my ~/.gnus file:

    (setq gnus-global-score-files
           '("~/Mail/all.SCORE"))

This does seem to work properly for the rule that matches the From
line - matching messages are highlighted, and when I type V S in the
summary buffer, it shows a score of 1000 for those messages.  However,
the "To" rule doesn't match.  I've seen in the documentation here:

  http://www.gnu.org/software/emacs/manual/html_node/gnus/Score-File-Format.html

... that "Scoring can only be performed on these eight headers: From,
Subject, References, Message-ID, Xref, Lines, Chars and Date", but
here:

  
http://www.gnu.org/software/emacs/manual/html_node/gnus/Scoring-On-Other-Headers.html

... says that one can get around this limitation by adding the following
to .gnus:

     (setq gnus-extra-headers '(To Cc Newsgroups Keywords)
           nnmail-extra-headers gnus-extra-headers)

... and then restarting Gnus and running M-x nnml-generate-nov-databases.

I've tried that (with just To and Cc), but the rule for the "To" line
still isn't working, even if I type V R to rescore the articles in my
summary buffer.

Could any possibly suggest how to get this to work?

Many thanks,
Mark