Telewest did this to my previous home (and gobbed it up with clear
silicone) while I was out. When I arranged to have cable fitted at my
new place I made a point of being there when the engineer came. I had
took the time to mark out the hole locations inside and out, and asked
him to drill from the external inwards. He said that he could not do
this as there may be cables behind the plaster. I laughed and pointed
out that the wall was recently built and was not plastered yet!

He said, oh in that case, there may be cables inside the cavity. I
looked at him and said, give me your F*@#ing drill and I will drill it
myself. And he made me sign a disclaimer.

dg