Why does a room at 19 degs feel colder on a cold dark night?

[adam2]

I am inclined to agree, though the new owners of the property consider the running costs to be "very reasonable" if compared to their previous home.
They keep it warmer than I would, though not to a great degree.
It is a very large home, and rather exposed to the prevailing wind.
Oil used has been "less than expected" but logs more than expected.
The otherwise unheated sun room or conservatory has an open fire, but this is very little used.
Like me, they have a large supply of small oak logs from a local farm at which several large oaks were recently taken down for building timber, with all the small bits used for fire wood.

[adam2]

DATA, in very recent heat wave.

Outside air--------------------------------30 degrees, briefly 32 degrees.
Fresh air supply--------------------------12 degrees average.
R.H. External-----------------------------65%
R.H. Internal------------------------------48%

Inside temperatures.
Master bedroom---------------------------23 degrees
Other bedrooms---------------------------23 degrees
Kitchen--------------------------------------24 degrees
Living room---------------------------------24 degrees.
Spare rooms---------------------------------26 degrees

Heating fuel used---------------------------zero.

Electricity used for air conditioning -----60 KWH/day (average over the hottest week, not typical use)
Total electricity used, all purposes-------100 KWH/day (also average over hottest week)
Net electricity consumed ------------------41 KWH/day (average over hottest week, after deducting PV production)

Total electricity used includes extra laundry due to visitors and extra electric hot water also.
New swimming pool heated by solar thermal, pumping included in general electricity use.
Domestic hot water heated by off peak electricity, and by surplus PV during daylight.