Pixellated pictures on live TV are often the sign of a signal fault. If you've rebooted your box once, then there's likely a fault somewhere, so call VM and report it. You're likely to need either an engineer visit or a replacement box, depending on what VM's remote diagnostics turn up. They can do these while you're on the phone to them.
If C4-HD has issues, how about... C4+1SD, ITV+1 SD, or C4SD? These are carried on the same broadcast frequency as C4-HD, so if one of them has signal issues, then they all should be affected,
First check the automated faults line for any existing issues in your area that could be the cause: 0800 5610061. If nothing's reported, assume VM don't know about your fault and you'll need to call them.
150 from a VM phone, 0345 4541111 from any other phone - choose the options for "I have a fault with my TV service"
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