November was unusually cold. It has recently warmed up a bit. I use about 14 times as much energy (gas) per day in frosty weather as in summer when I am just heating the water, you usage pattern may be similar.
Perhaps you have a large property, and if it is open plan a lot of heat will rush up to the ceiling.
Do you just have radiators, or UFH? Do you have a conservatory? Do you know the power of your boiler?
Go round and look at your radiators. Calculate, if you added the length of them all together, how long a radiator would it make? Count a double rad as one and a half times its length. How tall are they?
I expect you have thermostatic radiator valves. What number are they on? Are all rooms heated? Do you close the internal doors?
From the inside, can you see the exterior walls, or were they all insulated and boarded? Can you see the underside of the roof, or is there a loft and flat ceilings?
What do you know about the insulation that was installed?
Have you hot glass doors or glass walls?