Always remember when changing from a dynamic to a static IP address to kill the DHCP client daemon. So that is what happened to my default route, 24 hours after I first swapped the server into use. I knew I should have rebooted.

Hmm I wonder if the DHCP client can be made less aggressive in this regard, in an ideal world it would check if its settings were still in effect before deleting the current settings, and "/etc/init.d/networking restart" didn't know to stop it, because by then the other "interfaces" file was in place. More idiot proofing needed for me.