[Exim] Variables set in system filter not copied to user's f…

I'm testing a new exim 4.30 installation, and have a problem with
passing variables from the system filter to the user's filter.

I'm sending mail from the mail host itself, and it is being
delivered correctly. Both the system filter and the user's
filter are being run, and both create log entries (with logwrite).

The system filter is:
# Exim filter
logfile /tmp/system_filter.log
if error_message then finish endif
add 7 to n0
logwrite "$tod_log $message_id n0=$n0"

The user's .forward file is:
# Exim filter
logfile /tmp/user_filter.log
if error_message then finish endif
logwrite "$tod_log $message_id sn0=$sn0 n0=$n0"

A typical log entry in /tmp/system_filter.log is
2004-03-11 11:40:20 1B1OY0-0000t9-6z n0=7
and the corresponding entry in /tmp/user_filter.log is
2004-03-11 11:40:20 1B1OY0-0000t9-6z sn0=0 n0=0

Why is $sn0 not being set?

Thanks for reading this...

-- Malcolm