Thanks Graham
I have tried that, but when I do that I just get everything as a letterbox! If watching a film and I press either Play or Pause (and in that situation the ‘bar’ at the bottom of the screen has almost disappeared below the picture) the display will sometimes revert to what seems a correct format, but then sometimes jump back to the incorrect display.
I presume that there must be something included in a transmission that tells the display what format to use when in Auto. As a specific example, a McDonalds advert made the screen ‘go thin’ and then the following advert made the screen revert!