Pease can you try another source such as freeview, a DVD player or a games console and let us know if the picture is normal with them. This will let you know if it is the TV has developed a fault like a faulty backlight.
is there a another tv in the house you could connect your Virgin Box to? This could let you know if it is the box too.
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It resolved itself following another couple of reboots, so not sure what was causing it. Did the same initially on another screen I tested, so putting it down to the Tivo box or connections, but it's fixed now.