You will find that most utility companies employ sub-contractors and more so at peak periods, and people do not seem to realise this when obtaining quotes and services. But having stated that, in the end it would be the main company dealing with the final sale that would be responsible for resolving any issues, and in a speedy customer friendly fashion.
When we had our first set of central heating installed, it was no better than the photographs that you have shown. Trying to get sense out of the main contractor was extremely difficult, with constant arguments between ourselves. Even Corgi, who the contractors were a member, didn't want to know about the poor installation work. In the end we went to the small claims court, were judgment was found in our favour. The people responsible for the initial quote, gave assurances that everything would be 'put right' with compensation as a gesture of goodwill. That never materilised, and we ended up out of pocket, not only for the remedial work, but an independent report.
We have since had a refurbish exercise on the central heating via a government grant, and the work on completion was inspected by an independent surveyor. A number of item were found to be wrong, including parts covered by regulations (electrical safety links missing etc), and all this was completed to both our and the inspectors approval. We did allow one thing that was incorrectly located to pass (an upstair thermostat), but this was due to the upheaval of disrupting an household again.
The first incident and installation I would have wished on nobody, the second 'modernisation' was a dream come true!.
What still concerns me to this day, is how a company can provide sureties of workmanship and safeguards, by quoting membership to certain organisations, and at the same time of desperate need, some of these organisations fail the public and appear to defend their members.
You have stated "but we are now faced with the enormous task of fixing the damage"- you should not. Could I make the following suggestions. Contact your local Trading Standards or free Legal Advice Centre for advice. Check to see if NPower are registered with any organisation or arbitration service. If you paid for the installation via credit card or a special finance packet or agreement, then take the matter up with the finance provider. And write a polite but firm letter to NPower stating that you regard then fully responsible for resolving the issues to your satisfaction. If you have to take a small claims court action against NPower, then if judgment is in your favour, you should have a swift resolution, and not like me who was was fighting and losing a battle against a very much smaller independent company.