After some experimentation I managed to reproduce the problem he was having. He'd used DS1820 sensor ICs instead of suggested DS18B20.
I'd never really thought about the differences between the different 1820 IC's, but there are some, and unless you change the firmware for the clock, it won't support the basic DS1820 variant.
This article from Maxim sheds some light on the issue.
Anyway, after switching to a DS18B20 variant, the clock is now fully functional. Well done Gabriel.