There are better ways to store heat, but they need room, one method is water, but water is heavy, and bulky. My brother-in-law had a water store, it was used to store excess energy from using the wood burner in the evening, and also solar panels and also if required LPG gas boiler to heat the house through one combined central heating system, there were two large tanks so around 1.5 x 3 meters of floor space used, and very expensive to install.
The cupboard was allocated as the house was built, with the support required for such a high weight, personally I feel likely not worth the money to install.
So over the years the benefits of off peak have reduced, so you need to re-assess if still worth using off peak power, it may be better to use simple panel heaters as an when required, but the calculation is not easy.
This house not that well insulated, draft proof, but old double glassed windows really need changing for the new more energy retaining type, last house we lived in was Mother's old house it was very well insulated. With her old house, boiler would run in morning and the temperature tended to over shoot a little so switched off 10 am and would not switch on again until around 6 pm, this house the boiler runs on/off all day, so Mothers house would have stayed warm with storage heaters, this house would not.
If it were me, I would try an experiment, I would use a cheap 2 kW heater and an energy meter all simply plugged in, instead of the storage heaters for a week and see what energy they have used. Also note the total energy used, and compare with the storage heaters, as there are so many factors it is near impossible to calculate if cheaper using heaters as and when required, remember if you get rid of economy 7 the on peak power in the main is cheaper. Clearly it needs some hunting to find costs.
So if you can say 50 kWh used in week with panel heaters, and 70 kWh with storage then you can work out which is cheaper. Other wise it is just guess work.