I'm assuming that of the two heat sources the OP has, one needs to be
gravity run as I have. I would be interested to know how the heatbank
arrangement can be configured for this;

The heatbank is just a huge Dunsley Neutraliser. It is plumbed in the same
way. The only differences are additions.

(1) A pumped plate heat exchanger loop to provide mains pressure hot water.
(2) An indirect coil to allow modern sealed pressurised boilers to be used.
Gravity fed (or circulated) appliances go directly to the heatbank, not
through the indirect coil.
(3) An immersion heater (or 2) for electrical backup.

Christian.