Less common that the general light stalk failure, is intermittent failure of the 'flash' functionality on the headlamp main beams.
It may be that the headlamp main beams work fine in normal operation, but the pull stick to flash function doesn't work, or works only when you pull the stalk slightly instead of completely.

First, disconnect the battery - Remember to have your radio code to hand!

Remove the 4x T10 screws which hold the dash on.

Unclip the stalk, and remove the wires.

Gently use a thin watchmakers screwdriver to remove the cover from the main body.

Now be very careful when removing the cover, the 'cancel indicator' function has a little jockey wheel which locates on the back of the cover.
Carefully lift the stalk arm off the circuit board and release the small sprung conector.

The flash unit connections are shown below, use contact cleaner and a clean cloth/paper towel to clean the dirt and tarnish off these 2 connectors.

Reassembly can be a little fiddly because of the jockey wheel, but use the thin screwdriver to gently push the sprung wheel back, until you close the cover and it springs back to the indicator cancel arm.
Refit the stalk back into the car and test.