Just as a case of interest (?) , for 9 days very recently our boiler failed and the house was (inadequately) heated by various electric fires/heaters. An immersion did a poor job of heating enough water. Because I'm with Octopus, I submit readings late on the first of the month and get a statement within hours. I am thus able to not only keep an accurate record but to compare comparative months.
The reduction in gas usage and increase in electricity over these 9 full days of reasonable temperatures (for January) culminated in an extra £25 or so. Despite the unpleasantness of moving from reasonably warm room to cold areas and back to warm, and that these electrical heaters were nowhere near warming the internal structure like my gas c/h, and that there was inadequate hot water, which cooled quickly, I don't think that's too bad. but we economised as best we could.
Even though I'm on a fixed tariff, it makes more sense to me to jettison a 35 y.o. (+/-) ( now unreliable) boiler and have a complete refurb. of the system as the efficiency upgrade will incur greater cost savings (as well as more comfortable domestic conditions) because of the price rises which I copped some of when I fixed in mid September.