|Got it sorted.
I had to uninstall DW completely, making sure there was nothing left behind - registry settings, folders, that sort of thing.
I restarted my computer and reinstalled DW.
Installed the server behaviour through the DwZone Extension Manager and it showed up in the list. Installed all my other DwZone extensions and they are all available.
Still have no idea what caused this but at least it's resolved.