I live in a 2 bed house (on my own mind), storage heaters and immersion heater just like you, according to my electricity monitor I used 440KWh this month, 475KWh the previous month, so that would be around 1100KWh for 70 days. I've been keeping it quite cool though, I could imagine it's possible to use 3500KWh with the recent weather. How hot do you have the house?