This is a DHW only heat bank. So you will be having a diverter valve for
either CH & DHW, with the CH running directly from the condensing boiler.

My original intention was to use no diverter valve, just 2 ports on every
circuit. Now, I'm not so sure. I may install a diverter valve for the hot
water and use 2 port valves for the heating sub circuits.

What kW rating is the coil?

I don't know. However, the cylinder appears to be made by Telford, so I
might have a look to see what their standard coils are rated at. I suspect
it will be less than the boiler's maximum output, hence why I wasn't too
concerned about priority operation.

Christian.