What's More Effecient? Oil or Electric to Top Up Central Heating?

Having recently signed up to Bulb I note that the night rate is significantly lower than the day rate, this got me thinking that if I used the immersion in the thermal store to pre heat the water rather than the oil boiler it would be cheaper. I think this because of the thermal loss over the distance between the boiler and the thermal store, as well as the price of oil compared to the price of night time electric. Generally the house is heated by logs (I am a firewood merchant), but we use a Nest device to provide the controllability, linked to the oil boiler. So the question is am I better off using the immersion to heat the water in the thermal store over night, rather than the oil? Could I link the Next to the immersion? Any comments welcome.
Thanks
Daniel