Pi Screen limited to 1920 by RISC OS:-

Thanks William. It works:-) I didn’t need disable_overscan=1
I’m not sure if the order is important but this order worked for me:

hdmi_group=2
max_framebuffer_width=3840
max_framebuffer_height=2160
hdmi_pixel_freq_limit=400000000
hdmi_cvt=2048 1152 60 3 0 0 1

Just had a thought I thought I’d read that to use cvt you to specify hdmi_mode=87

So I’ve just tried remming out the cvt line and it still works!

hdmi_group=2
max_framebuffer_width=3840
max_framebuffer_height=2160
hdmi_pixel_freq_limit=400000000
#hdmi_cvt=2048 1152 60 3 0 0 1

The only documentation I can find for CVT is here http://www.raspberrypi.org/forums/viewtopic.php?f=29&t=24679

So I conclude that as EDID gets a 2048×1152 display correctly but specifying it in a mode file doesn’t MDFs can need more information than just x & y.