Why won’t Virgin Media complete my broadband installation?

The company won't phone when promised or provide an explanation

I have been trying to switch to Virgin broadband because it is by far the fastest in my area. The installation was booked for mid-August and, as at that point a full connection was not possible, we were connected to our neighbour's cable with their consent.

Since then, Virgin Media claims it is unable to complete my installation or deal with problems with the service. Apparently its system shows the installation as complete and our account as active. In addition, the company has repeatedly failed to phone when promised, turn up in the agreed time slot, arrive at the right address or provide any explanation for anything. I am unsure what to do next, other than to send another complaint.