well having a quick peek at the operation flow chart on page 18 of
this manual (dont know if its your exact model)
I'D say its possible either the flow switch is stuck closed
or the DHW tank stat isnt satisfied
Matt
Turn down the DHW stat to off, not the switch, and if the pump stops, then it is the DHW stat faulty. If it still runs, then flow switch must be stuck.( If a leak on the heat exchanger, it must be one hell of a leak to operate the flow switch.
Although there is always the possibility there is a fault in the pcb or the relay. Follow the fault finder with your multimeter.