Finally the disk space being used by uninstalled packages forced me to do something about the Xorg 7 upgrade.

After upgrading to Xorg 7, nothing graphical worked as I hadn't installed the drivers for my video card, but this time installing the drivers was sufficient "apt-get install xserver-xorg-core xserver-xorg-video-vesa". I tried the ATI driver, but this led to some minor issues with my Rage XL (on board) video, where the backdrop to "desklets" were being too agressively cached, rather than refreshed when wallpaper-tray changed the wallpaper image.

Relatively painless, although finding nautilus wasn't installed at the end of it was a bit of a shock (no desktop icons, no file manager app), but one command, and my sid desktop is now ready to rock and roll.