My apologies for the delayed response. I'm not sure if updating the firmware or my trying hdmi_mode=5 rather than 16 but I've got Raspian desktop in full glorious 1366x768. In case this helps anyone else in the future I'm using the following settings in config.txt
framebuffer_width=1366
framebuffer_height=768
hdmi_force_hitplug=1
hdmi_group=1
hdmi_mode=5
hdmi_druve=1
After transferring these settings to LibreElec's config.txt, I'm very pleased to say we have lift-off.
Thank you both for your time and trouble.