The hot water tank loop and the heating tank loop do tie in together at the return. I don't see a check valve anywhere in the hot water tank loop. There is a circ pump but with no check valve installed.
With the ball valve fo the auto feed closed the only place domestic water use can influence the system is through 1-the hot water outlet on the tank and 2 the cold water inlet on the tank. The pressure spike must be transfering from the inner tank to the outer one and into the heating system. Maybe the pressure is pulling the swing disk in the return check valve. The banging is followed by an echo...BANG,Bang, bang. which is why I thought it may be that check valve.