P.S. to Summer increase in line loss.
Some years ago results were published of experiments with boreholes at GPO Leafield. These showed that at 10 m depth the cyclic variation between summer and winter was slight ( > 1 degree from memory) and
out of phase with the surface temperature. Day and night variation ? long gone.
Which is why our ancestors, who weren't stupid, dug cellars to preserve food !
A pity BT ducts are not that deep