10.5 updates on a 10.4 Server

Hi all, I'm currently having to run our main servers on 10.4 but I now have almost exclusively 10.5 clients. It appears that the 10.4 Server won't download 10.5 updates (or anything that relies on 10.5) so I'm stuck. I've looked all over the Server Admin application and any preferences I can find with no joy. Anyone know how to get around this? // Tony