Hmm turns out resetting the TV got rid of it. Still no idea how I managed to get it to appear.
Have not connected this TV to any aerial or dish, so it complains about lack of channels, but when I switch to an HDMI input (tried 1 and 2) I get the correct picture, but with a large and annoying overlay showing "0" on the RHS that I cannot make disappear.
Other (non-channel) sources (i.e. plex, netflix) are fine.
Any ideas how to get rid of this irritating eyesore?
Photo here:
https://drive.google.com/open?id=0B7ikxq_p_3y3WWI1Rmk3QWxYNTBsZnpqNDRrSHBwRVI3U2RV

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