Docc is right (again). You need an electronic flasher [21] which will flash the indicator lights irrespective of current draw from the lamps. I did this, and found another problem.
The instrument panel [6] turn signal light is connected to both left & right indicator light circuits, so if you have all four indicators fitted with LED lamps, all four lamps will flash when you indicate left or right turn. That [6] indicator provides a path to ground via the side opposite the side you selected. The fix is to replace the little incandescent bulb in the [6] panel with a facing pair of LEDS, with a common ground.
Until I follow my own advice, I have only LEDs in the front indicators, same as Docc's bike.
Note that I left out a wire in the diagram below. The red wire should go from the connector to the RH indicator lights.