Drain issues - how severe?

After having drain problems, we got Dyno-Rod to do a survey on our drains. They found four of the seven pipes have bellied, with water and waste sitting in them. They say this could be caused by subsidence or poor installation.

We have complained to the developers who are looking into it, but already using phrases like "it's relatively common", and I get the impression they are not going to take it seriously.

How much of an issue is it? I've threatened to open an NHBC dispute resolution but I don't know how hard to push.

Many thanks for any advice.