1920 x 1080 @ 60Hz Monitor Image format provided by RasPi, as above. Reported by monitor and by RasPi.
Reduced image size.
lxde -> Preferences -> Monitor reports 1824 x 984
Solution (Information obtained from above references):
This put the image at full 1920 x 1080. This also put the image right to the edges; this is too close. Edited the config.txt to read.
disable_overscan=1 overscan_left=5 overscan_right=5 overscan_top=5 overscan_bottom=5
This gave the image a 5 pixel border.
Occasionally the RasPi reboots with a centralised colour reference square, but generally boots normally.
References above also give information as to disable the RasPi HDMI Hot-Plug Detection which will enable the HDMI output even if no Monitor is detected.