Do you have daily figures for gas consumption for some cold days, say around Jan 11th? Divide by 24 to get average heating power. This can give a good estimate for how much heat is required, though you’ll have to take into account the efficiency of the gas boiler (maybe 70-80%?) and the wood burner. Octopus would be hard pressed to argue against historical consumption.
Better still if you can find temperature records for those same days, and plot them on a graph.
Also, give Trystan’s new Simple heat loss tool a try, though it can only be a rough estimate.