I see you have swapped HDMI cables around, but have you tried a different HDMI Port?
Is the 360 connected directly to the mains or are you using a splitter?
The same question with your AV amplifier, also is your AV amplifier connected via HDMI, eArc or optical cable? Is the AV amp connected directly from the 360 to your TV, or directly to the 360?
To set up the AV amplifier you need to go to Home > Settings > Audio & Visual > Pair Remote With Devices. and follow the onscreen instructions.
You also say, "The picture format defaults to 4.3 every time we switch the TV on." - is that the actual picture on the TV itself via terrsetrial TV or only via the 360's HDMI connection?
Have you tried factory resetting the 360 box?
To factory reset go to Settings > System > Factory Reset. It's actually misnamed and only a partial system reset and not a complete factory reset. You are given 2 options
Choose Keep Recordings, that means you won't lose your recordings and any planned recordings or series links you've set up. You will have to sign into your apps such as iPlayer, Prime, Netflix and ITV Hub again. Also Match frame rate and Standby power consumption will go back to their default settings if you have changed them.
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