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 this will need VM to intervene. You're likely to need either an engineer visit or a replacement box, depending on what VM's remote diagnostics turn up.
It won't alter the end advice, but are you watching Sky F1 in SD or HD?
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 ideally 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". Alternatively you can wait on here for staff to respond, which may take a day or so.
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Watching on SD, but I found quite a few other (non Sky) channels affected too.
Previously turned the box off and on with no improvement, but yesterday tried reset via the menu and pixellation has gone away. Don't really understand, I would have thought a power cycle would be a potentially deeper reset than via software, but currently all is good.