I've attached a separate screen via the displayport, and the radeon driver
loads and I get the second monitor as a secondary screen, from which I can
access kscreen through the settings dialog and then set as the primary.
Throughout this time, the secondary display is working but is dark in
comparison to what I would expect, but the primary screen, the imac, is a black
screen except for tear type artefacts like barely visible equidistant lines
floating upwards.
I would guess that the edid is reported incorrectly, as the imac screen is
reported in the settings dialog as 'laptop'.
I'm going to try obtaining and setting an edid etc now.