Your economy 7 immersion heater is the bottom one and should be wired (if done correctly) to come on via a timer at cheap rate times.
Your storage heaters should also be wired to the same circuit and set to come on with the timer.
Only appliances connected to a cheap rate meter will register as nite units and be cheaper.
Suggest you ask your electric company to check if your supply has the correct meters to take advantage of E7