Well, the Pi turned up a while back. Not owning an HD telly meant that a monitor needed to be found.
My old HP F1903 has been in use as a second screen on the iMac for some time and thankfully has a DVI port in additon to the VGA used by the Mac.
How hard can it be?? Cheap HDMI -> DVI-D cable from Maplin. (No such thing - its more expensive than the PI!)
Cheap HDMI -> DVI-D cable from Amazon then - that's better.
Unfortunately, a lousy picture - green lines and rubbish all over the screen...
Anyway - the upshot of loads of hours mucking about with the config.txt settings; reboots and using an even more ancient Phillips CRT monitor from my Atari XE days
and contrary to the instructions in Eben's new eBook manual; the following settings worked for me:-
# Config.txt
# Using cheap HDMI to DVI converter cable to HP F1903 Monitor
# Best mode appears to be Mode 36 (1280x1024 75Hz)
#
# HDMI type DMT
hdmi_group=2
#
# Drive is DVI Sans Sound
hdmi_drive=1
#
# The following modes are supported by the HP F1903
#hdmi_mode=4
#hdmi_mode=5
#hdmi_mode=6
#hdmi_mode=9
#hdmi_mode=10
#hdmi_mode=11
#hdmi_mode=16
#hdmi_mode=18
#hdmi_mode=35
hdmi_mode=36
#
# So we get a bit of border round the edges
#
disable_overscan=0
# Boosting the HDMI Signal to get rid of green line interference
# Anywhere between 1 and 7
# 2 seems to work, 3 is better in LDXE
config_hdmi_boost=3
#
So there you have it. A working monitor for the Pi, which I can use a t the same time as the iMac. Press the -/1 button for iMac's VGA screen and the +/2 button for the Pi's DVI display.
(Don't forget to ser the monitor to "Video Input Select/Auto Detect")