On a purely practical and non-scientific test over the last two winters I've
left our heating on constant and turned the thermostat down to 20 at night.
(I don't think the scale is very accurate as we run it at about 25 during
the day - and the house is seldom too hot.)

It's a 1988 4 bed detached and we have no TRV's. The kids used to wake at
night and it was far more pleasant to deal with in a warm house, I also
appear at home at all hours of the day after site visits to do my
paperwork - nice to return to a warm house.

The result has been a very marginal increase in our gas bills, though not
enough to increase our monthly direct debit. The heating seems to be off for
the vast majority of the time during the night, and when I'm sitting at home
during the day. The seemingly endless hot water is also blissful!