For anyone who wants to follow along, I'm doing the V0.2 implementation of OpenTRV (http://opentrv.org.uk/) here:
https://sourceforge.net/p/opentrv/code-0/HEAD/tree/trunk/Arduino/V0p2_Main/
See timetable:
This is a bare ATmega328P programmed to run at 1MHz from its internal RC oscillator so it can run off 2xAA NiMH batteries (~2V) and uses a 32768Hz crystal (with timer 2) for accurate time-keeping.
Rgds
Damon