After seeing the severe day-to-night temperature variations in the board room, I decided to monitor a room in another corner of the same floor. I used a small EnviroMon system, separate from the permanent system that we have installed in the rest of the building. The graph shows that the upstairs spare room behaves in a similar way, suggesting that it's the construction of the outer walls that's to blame rather than any oddity of the air conditioning or building layout.
Today we investigated the erratic heating the MD's office and board room, and found that the temperature sensor was located in a cold draught coming through the ducting from the roof void. We moved the sensor away out of the draught and the temperature stabilised.