I'm rather curious about that as well. We're not in our house yet, but when visiting to see whats going on I have noticed that even on grey cloudy days the house does warm up a bit. There were a few cloudy days in a row last week and the maximum temperature for those days was only 17C and mins around 10-12C. But when I took around a thermometer in the afternoon it was 20C inside in the living areas. This is with the garage and front door left open by the tiler (though at the other side of the house). The other bedroom areas are without much north facing light and they are noticeably cooler. But then there are some windows still cracked open by the painters too
I suspect it must get quite a bit cooler at night - there's no blinds or curtains yet obviously. But it would appear that there is still quite a bit of heat available from the sun on cloudy days. We'll know for sure when we get handover and I can put a temperature logger in.