Pixellated pictures on live TV are usually the sign of a signal fault - one box reboot is a good idea, if problems persist, then there's a fault somewhere that VM will need to rectify. You've not said whether you pay for Sky Sports HD or not, it will only make a difference to enable VM/other users to check which other channels are carried on the same frequency and therefore you could check those.
If only one of your boxes is affected, that rules out an external fault and suggests the problem is either with the box itself or your internal cabling. Try swapping the boxes around and see if the fault moves - if it does, the box is clearly faulty. If the fault stays, the cabling is likely the problem.
At that stage, you can report the problem to VM who should arrange an engineer visit to replace whatever kit has been proven faulty. If you wait here for a few days, VM staff should respond. Otherwise you can call into CS (150 from a VM phone, 0345 4541111 from another phone) to report it immediately.
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