I've changed the spam checking script so it checks all inbound mail even if they already have 'X-Spam-Status' headers. It strips all exising X-Spam-Level, and X-Spam-Status headers before the mail is sent to SpamAssassin.
Hopefully this will prevent some spam that has been getting through.
Removed headers are available in "X-Newswall-Removed-Header" if you're interested.