When I switch the indicators on (left or right) the main beam indicator (green light) flashes too. All circuits and switches appear good, what have I done wrong?
The main beam indicator is usually BLUE. The MKIII Commando uses GRN for the Neutral indicator. The ORN indicator for the indicators has a GRN/WHT one one side of the lamp and GRN/RED on the other side of the lamp. The ORN indicator warning gets its EARTH through the 'non flashing' side of the indicators. It would seem with this funny fault that the main beam warning (should be blue) has picked up its earth from the Indicator warning light. What bike is this?
I recently had a similar problem on my Mk3 Commando and gradually traced it back to a bad earth under the tank. There are a couple of socket connectors that I cleaned and stopped the erratic flashing of other indicator lights. Look at your wiring diagram to check where the earth wires are.
.. are very common causes of things like this - such as stop lights coming on with indicators etc.