Something sounds off in that system: Maybe the flow temp as you suggest, also worth mentioning if you have balanced your radiators? I found this made a big difference when I did it a few years ago. Also making sure TRVs aren't stuck open (I move them from time to time during the summer months if I remember). Obviously insulation helps too, double glazing and thick curtains, etc, plus the obvious (though I struggle sometimes with my family...keeping the blooming doors shut.
).
For comparison I put my heating on early yesterday at about 4:40pm and by about 6pm the living room had gone from 16c to 20c (the thermostat setting) as I was checking. I can see that after that time the gas use dropped for each half hour after that too. By 7pm the only gas used was my son having a shower, then it kicked back in from 7:30 to 8pm. Tiny amount of gas between 8 and 8:30, then another spurt before the timer kicked off at 9pm. I think some of our evening meal was cooked on the gas hob too:
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I think the radiator in the living room is 10000Btu (room is 6m x 3.8m approx), but I can't remember for sure. I've gradually replaced them as I decorated each room.
I monitored the room temp each hour after that out of interest: It lost 0.5c each hour, so by midnight was 18.5c (I was watching the end of a film which finished shortly after midnight).
If you look at the morning use I only have the heating on for 50 minutes from 8:10 until 9am since we're retired and I prefer the house to be warm while I'm having my breakfast. You can see that by the second half hour it's using much less gas as the house warms up.
I hope that's of use, just for comparison: 4 bed 1970s detached, but with cavity wall insulation, extra loft insulation and double blinds/curtains in the living room (a large window, 1 x French door & a patio door to a conservatory).