Not quite everything...
You can have an unpressurised mass of water in a tank, which you can heat from a pressurised primary via a heat exchanger (if you want to do it fast - you could also do it via a conventional indirect coil) - you have a secondary pump to take water out of the bottom of the tank through the HE and back into the top to extract the heat from the boilers primary flow.
Then on the extraction side you have a similar arrangement with a big plate HE. Again a secondary pump passes hot water from the top of the store store through the HE and back again, to heat incoming cold water from the mains.
In effect you get a combi type of effect where the heat comes from the heat bank. This gives you mains pressure hot water, at any flow rate the mains can deliver rather than being limited by the rate at which the boiler can supply it.
You could do it with less hardware using a vented primary boiler (no need for the charging HE, the boiler can just circulate its primary water into the cylinder. However that does not suit my application since I have no loft space for header tanks[1], and much prefer sealed primaries anyway.
[1] There is currently one cupboard suited to the location of the header (and that is where it currently is), but I want to turn that into an ensuite shower, so both tanks in it need to go.