Hi John, please see my earlier reply.Yes, more or less correct, but a setting of 8.5C equals to your measured temperature of 12.5C so maybe repeat your test and if the same I would set the stat almost fully clockwise to its original setting of "2" which means that the stat will operate when the temperature is 5C?.
almost fully clockwise"2"---it might meant by the installer that "was disconnected"?