If you asked the nearest seven-year-old (before you get arrested) then they would probably say why don't you take a meter reading of your twice a day use, then take one of your all-day use, and take one from the other. I doubt if the seven-etc could spell initiative, though.
I don't have a meter as I don't have gas, but - being interested in any theory that might produce lower fuel bills - I took a few timings of my oil boiler this morning. With the c/h on it fires for just over two minutes in every five, approx 26 mins an hour. On the twice a day setting, allowing for the two 40-min warmups in the morning and evening, the boiler fires for 4.5 hours a day. As the boiler specs say 3.2 ltrs an hour that's 14 litres a day, just about what we use.
So if we left the boiler on all day it would not have the afternoon warmup, but seven hours of 26 mins during the day. The boiler would be on for a total of just over seven hours a day, using 22 ltrs. So twice a day it is.
We also have windows thrown open during the day, and wandering in and out, so I would be a real miserable git if the boiler were chugging away to warm fresh air.
Rgds.