Yes but if the immersion is powered from one of the cheap rate power lines it will be much more cost effective than the boiler in cheap rate times and it may, may, also help the water being heated in the top of the tank by the boiler heat the whole tank because it will set up a convection current in the tank. The immersion heater is all bar the shouting 100% efficient in that all the heat goes into the water, none is wasted - until it gets to the outside of the tank and leaches away into the air.
If it is a 3 kW element and it runs for 3 hours, that is 9 kWh.
You must take very long showers, or the heat (the old word for energy) is going somewhere else.
Re the above picture. Isn't it limited to 65°C now, what the reset is for?