Thankyou so much, its now basically working.
Here is my config.txt:
gpu_mem_256=112
gpu_mem_512=144
hdmi_ignore_cec_init=1
#disable_overscan=1
start_x=1
disable_splash=1
overscan_left=48
overscan_right=16
overscan_top=16
overscan_bottom=16
hdmi_force_hotplug=1
hdmi_group=1
hdmi_mode=21
hdmi_drive=2
However, I am having problems adjusting the overscan. When I change overscan_left, I can see the effect on the OSMC splash screen. Here it is with overscan_left=16:
You can see there is no black border on the left side of the splash screen.
Here it is with overscan_left=48:
Now we see a black border on the left of the splash screen. The splash screen is quite well centered.
However, once the OSMC menu appears, the effect of the overscan setting seems to be ignored. With any setting, overscan_left=16, overscan_left=48, even overscan_left=100, the OSMC menu looks exactly the same, with half the OSMC logo cut off and half of the “V” in “Videos”:
Can anyone suggest how to force the OSMC menu to respect the overscan_left setting please?