Halstead High Ace - DHW not turning off?

After turning off the DHW tap, my Ace High boiler stays in Hot Water mode - this also heats up the radiators. Some time ago I had to replace the DHW flow assembly diaphragm because the hot water would not turn on - could this be related? The operator pin that activates the DHW flow switch seems slow when returning, therefore keeping the hot water on. If not the diaphragm, could it be the diverter assembly that needs a replacement service kit?