Triumph for some unknown reason decided to run the full load of the headlight thru the ignition switch which eventually get to hot and melts the solder there by no power to bike motor as and you are walking. It is an easy fix if you are handy with wiring and a soldering iron or you can buy the "Eastern Beaver headlight relay kit" which draws the power to the headlights directly from the battery via two relays (1 hi beam 1 low beam) there is only a about 3 amps going thru ignition switch instead of 30 amps, side benefit is the your lights will also be brighter. I made my own as have others it is not hard and cheaper than EB kit
Oh and another Welcome from Auss