[Exim] Filter file query - freeze

Hi

I have a rather large filter file to (attempt) to filter out spam
messages.
The basic idea is

if
$header_from contains "spammer@???" or
$header_from contains "anotherspammer@???"

then
  log "RULE 3 From: $header_from:"
  log "Subject: $header_subject:"
  log "To: $header_to:"
  log "Reply to: $reply_address"
  log "Date: $header_date
  log " "
  save /etc/exim.filters/caught
  mail
        to postie@???
        subject "Spam (Rule 3)"
        text "We got a spammer ! Subject: $h_subject From: $h_from"
  seen finish
endif

This works fine, but I would like to freeze these caught mails rather
than save them to a file, so I can check them out and thaw them or bin
them.

If I put a freeze line in the if statement and run a filter check on it
I get this error

Filter error: unavailable filtering command "freeze" near line 122 of
filter file.

The docs say that this is perfectly OK in the system filter file which
this is.

Any ideas what I'm missing here?

Thanks

Iain