Thank you for responding @davidh007 and no problem.
We just need to check what resolution the set top box is broadcasting at. Please can you go to:
Home > Help and Settings > Settings > Video Output.
What number is next to the video output format? If it is not already on the highest resolution, can you please try selecting change settings and seeing if a higher one is available. Also does the selected format have "not supported" next to it at all?