Gas Meter displaying XXXXX.XX

macman

350% is not theoretical, it is the approx difference between the EPG capped unit rates for gas and electricity. The comparison is based on using the same number of kWh, regardless of fuel.
Obviously if you choose to heat one room with electricity rather than a whole house using gas CH, then 350% would not apply, but then you are simply not comparing like with like.
I could have said 'in the meantime, using electric heating and hot water is costing you 350% more than gas for the same amount of heat energy, but that would just be stating the obvious.
If your boiler heats the hot water for an hour at 15kW output, you are inputting 15kWh of heat. In the same hour, your immersion heater is only inputting 3kWh. You'd need to run the immersion for 5 hours to get the same input.