I went back to Icaros 1.1.2 and installed it last night. It has as separate entry in the Grub menu for NVidia Graphics. If I select that, native graphics works and all the resolutions are available and it's noticably faster.
I then tried Icaros Desktop 2.2.x (can't remember exactly which one, but an early one) and it doesn't work, but now there is no separate grub entry to pick for NVidia graphics. So something changed between the 1.x and 2.x releases. Was there a different nvidia graphics driver released at some point?
It's really a pity
as the newer Icaros release are of course far better.
Darren