Average annual UK household elec usage: 2900kWh (are you likely to be significantly different from average?).
Household elec cost (assume all consumed in the day, do you use much household elec at night?);
2900 x 35.06 = £1,007 (shell)
2900 x 44.35 = £1,286 (octopus)
So, difference of £281 on...