Eliminate a box software gremlin by trying a reboot first.
If that fails to resolve, pixellation is almost always the consequence of a signal level issue somewhere between the streetcab and your box. BBC4-HD & Sky Arts HD are carried on the same broadcast frequency, along with CBBC-HD & CBeebies-HD (among others).
If those channels are also affected, you need to call it in as a fault - it will only be permanently rectified by VM intervention.
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