Thanks again - received, installed, and working.
:)
Debian just worked with no issues (using built-in drivers), whilst for Windows only the DVI port worked initially - wouldn't even send a low-res signal over either VGA/HDMI ports.
After maybe half an hour of waiting for an update (which spent most of that time at 20% with no indication of progress) and the driver download apparently stuck behind it sitting at 10%, it eventually completed the update and installed the driver, but still seemed not to work for a couple more minutes, then suddenly the HDMI screen blinked into life.
Anyhow, a couple of restarts later, both screens have correct resolutions, no flickering or glitching, menus look as they should, etc.