gravity system

I was about to change my Drayton LP722 to a Wiser model but just discovered the Wiser does not cater for gravity systems. Although the water heating is theoretically gravity, because of the distance and height difference between boiler and hot water tank it needs to be pumped to circulate...

Initial Plumbing set-up: Loft: Cold water tank with 1x 22mm feed to HWC, and tee-ing off this 22mm just after it leaves the tank is a 15mm HEP feed down to the Cold side of the shower pump under the bath on ground floor. (there are no other Cold feed off the tanks as all the cold taps are mains...