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It's normaly for each property to have only a single cable feed from the street. This cable is then split to serve the various boxes, either outside or inside the property. However when splitting cables the engineer should check signal levels are in tolerance. Sounds like that bit was missed, and if the cable wasn't split correctly with a decent electrical joint it can introduce interference whcih degrades broadband and TV.
Sound like your installer may have bodged it. Poor installation doesn't do the customer or Virgin Media any favour.
Hopefully the engineer comong on Saturday will take a lot more care.