800 litres of oil as the primary heat source for a 5 bedroomed house sounds to me untypically lower than any average.
Perhaps ubasses' house:
1) Has an incredibly high level of insulation in ceilings, walls and floors coupled with triple glazing
2) Ubasses' lifestyle is able to tolerate a comfort temperature in the property of less than average - say of 20 degrees Celsius - for some or all of the day.
There's nothing too difficult in understanding this. 'Heating' the inside of a house (to whatever one accepts as 'acceptable') depends on three factors: -
the relative difference between the temperature inside and out
the level of insualtion put in the direct way of preventing the heat inside from escaping to the outside and lost forever
the efficiency of the devices in converting fuel into heat (and Agas are not good compared to other sources whatever folks might claim).
The final factor that drives the 'cost' is the relative cost difference between the different fuel sources, with peak electricity, tankered-in gas and oil being towards the top of the list, and piped gas at the bottom.