Hi oxfordmark,
That is a bit of an odd one.
Just to confirm this is on a TV360 box? Is the internet to the box connected via ethernet or WiFi?
Is the box connected to the TV with a virgin supplied HDMI cable?
Sounds unusual but I would try it on different HDMI ports on the TV (1,2 AND 3) to see if it changes.
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I work for Virgin Media - but all opinions posted here are my own