Ok
So my hunch was correct; there is definitely something strange going on with the official driver package.. The "corporate install" package works just fine; you just need to use the MSI packages from the CLI and it all goes in fine, then install the drivers using the pnputil command, documented here:
http://displaylink.com/corporateinst...UsersGuide.pdf
And you are away.
I now have four screens, busting the stupid 3 screen limitation of the Intel HD4400 Chipset!