Heating on low constantly or higher for short bursts?

[SickOfTheSpiders]

For the last two winters, we've had the heating on 18 all night and 21 in the day due to having babies in the house. I was very surprised to find that our usage was actually very slightly lower than it was previously, when the heating was only ever on (around 21 deg) for a couple of hours twice daily.

So now I'm in a quandary - it seems madness with the current situation to have the heating on all night! But I'm wondering if just leaving it on 18 all day and night would actually be the most cost effective thing to do? What do you think?

Ours is never on overnight and is on am and pm for short periods. It's supposed to be a myth that it's cheaper to leave it on low rather than using a timer and/or thermostat.

Why don’t you do a comparison. Take a meter reading, and do a week of on all the time and then repeat but with it only on a couple of hours a day.
This way you will have a much better idea of what costs more