Are you crystal clear that if you are swapping one electric night storage system for another, it is IMPOSSIBLE to have any true gain in energy consumption? This is because ALL electric night storage system convert 100% of the electricity going into them into heat. (The issue with electricity is that it costs far more than virtually every other form of energy per unit of heat).
The ONLY cost benefit you might expect is if the outer insulation/flap system is better on new rads, it may be more able to retain the stored heat inside the unit until you want it out - to warm the room. This potential benefit is more likely to be of value if you are out of the house by day and want the majority of heat released in the evening.
That isn't to say it isn't worth replacing old rads - as new ones are smaller and more stylish - just don't be conned that it might save you money on heating. Storage rads are NOT like replacing an old inefficient gas boiler for a new one.