This project is now completed. The system has been working for 4 weeks without any problems.

I have been asked by a friend to build something similar to remotely control the heating in his holiday home, and I feel that a re-design is needed to make it more generic. Therefore, a more generic (and simpler) version of this project has been reborn as https://hackaday.io/project/25572-relay-controlled-by-phone-call