If a reboot hasn't resolved pixellation problems, and you've checked connections and HDMI cables then you have either a box fault or a signal fault, probably the latter.
Have you tried signing into Service status at the top of this page? It shows any local faults and you can also run a test against the box. By doing this VM should be able to try and connect to your box and determine what the problem is. Sometimes this results in them sending out a replacement and/or engineer.
If that doesn't work then you need to contact the Faults team on 150 from a Virgin landline, or 0345 454 1111, options 1,1 and 2 (Faults) butbest to call at 8am to avoid call queues
When calling Faults hopefully the system will recognise if you've used the online faults checker and place you into a queue for an agent to pick up.
If not and you get the automated response asking you to press 1 for the link to online support, ignore the options and don't press any buttons then you should be placed into a queue and an agent should pick up the phone and resolve the problem.
You could also try the text messaging service. Just send a text with a description of TV Faults to 07533 051809. Your message will go into a queue and a representative will respond to you as soon as possible.It may take a few days.
The Service you do for others is the rent you pay for your room here on Earth - Muhammad Ali