The best system on the market (in my opinion) at the moment are the
Thermaflow electric combi-boilers. You get wet central heating with them too. Basically they are a storage heater with 4 immersion heaters.
Interesting system. One that actually makes a great deal of sense as it uses predominantly off peak electricity.
Even better, reading that, Scottish Power do an off peak tarrif with 18 hours of cheap rate a day
Economy 2000 | Scottish Power | Thermal Genius
And if you combined it with a solar feed as well, even better.
I'm surprised it works so well though. Years ago, when refurbishing my last house, I considered trying to make my own "storage boiler" with a bank of hot water cylinders heated from immersion heaters overnight on economy 7. I concluded that it was impractical to use water as a heat storage medium for whole house heating as the number of cylinders of water needed was just silly. However I'll bet this works on the fact that with an off peak tariff like E10 or Economy 2000, it doesn't have to keep stored heat for so long, so not as much storage needed as the system I thought about.
The servicing is important with unvented systems. It's important to make sure the over pressure and over temperature relief valves are working. This is much more of an issue heating directly by electricity, as a failed thermostat can cause the water to boil.
But I am surprised the storage cylinder needs to be unvented. I'm increasingly seeing "heat store" central heating systems installed. These work like a hot water tank "inside out" The boiler heats the water in the tank directly (not via a heat exchange coil) and crucially these are fed from a header tank, so the cylinder is vented. Then the water take off for heating comes direct from the cylinder, and it's only the hot water, that comes via a heat exchange coil in the tank, that's at mains pressure, so no different to any mains pressure pipe.
So if someone could make one of these based on the vented heat store cylinder that would be even better as there would be no pressurised tank and no maintenance liability.