[exim] Exim and Mailman, group issue on Debian (squeeze).

I've spent a frustrating few days trying to make Mailman work with Exim.

When post a message to a list, Mailman produces this error :

Group mismatch error. Mailman expected the mail
wrapper script to be executed as group "daemon", but
the system's mail server executed the mail script as
group "Debian-exim". Try tweaking the mail server to run the
script as group "daemon", or re-run configure,
providing the command line option `--with-mail-gid=Debian-exim'.

Yes, the appropriate Exim config looks fine to me.

04_local_mailman_macros :
MAILMAN_HOME=/var/lib/mailman
MAILMAN_WRAP=MAILMAN_HOME/mail/mailman
MAILMAN_USER=list
MAILMAN_GROUP=daemon

450_exim4-config_mailman_router
  mailman_router:
    debug_print = "R: Mailman rule $local_part@$domain"
    driver = accept
    require_files = MAILMAN_HOME/lists/$local_part/config.pck
    local_part_suffix_optional
    local_part_suffix = -bounces : -bounces+* : \
                        -confirm+* : -join : -leave : \
                        -owner : -request : -admin
    transport = mailman_transport

40_exim4-config_mailman_transport
  mailman_transport:
    driver = pipe
    command = MAILMAN_WRAP \
              '${if def:local_part_suffix \
                    {${sg{$local_part_suffix}{-(\\w+)(\\+.*)?}{\$1}}} \
                    {post}}' \
              $local_part
    current_directory = MAILMAN_HOME
    home_directory = MAILMAN_HOME
    user = MAILMAN_USER
   group = MAILMAN_GROUP

Using this config, MAILMAN_WRAP should run as MAILMAN_GROUP right?

Googled myself in circles, and have tried the config suggested in
README.Exim4.Debian.gz, with no better results.
I'm beginning to feel a bit sad about this, can anyone offer any useful
advice?

best regards

Matt