[Exim] Exim 3 System Filter - Spamassassin

I want to create a system filter that will direct/collect all the SPAM
email coming in one email folder (/var/mail/SPAM)
From the spamassassin documents I have the following system filter file
in /usr/local/exim/spam.filter

if
    $h_X-Spam-Status: contains "Yes"
then
    logwrite "   => junk : SPAM"
    save /var/mail/SPAM
    finish
Endif

Now I have to edit exims configure file and this is where I'm not sure
where I should put a message_filter command since I already have one in
the "Main Configuration Setting" for basic virus scanning (Exim system
filter V1.16)

Do just add another message_filter=/usr/local/exim/spam.filter line, or
do I need another redirector/transport just after spamassassins entries.

What about the message_filter_file_transport ??

Thanks for any ideas, because I obviously, don't have any :-o

Leigh