And depending on the heat bank system you can have 10 bar pressure, not 3.5
as on unvented cylinders.

Well, it is limited to 6 bar. If your mains even approaches this pressure,
you need a PRV in case of natural variations. I haven't measured mine, but
I'm sure it isn't that high. I'm on a hill and the flow rate is about
average. No banging from the taps, either when turned off.

This will combine the energy of the thermal store and the power of the
boiler, in effect making the cylinder larger.

No need. The cylinder is well overspecced, getting on for two baths with no
recovery. I can wait for the cylinder stat to kick in, which it will fairly
quickly, as it is right at the bottom. It is an indirect system, so I can't
directly combine the boiler output without a rats nest of expensive zone
valves, flow switches and additional heat exchangers. It is indirect as I
need to run a pressurised primary system.

Christian.