Step 4: 'Ears how you succeed
Once I'd tested one channel (and make sure it cut out when I pluggedin the jack plug), I repeated the process for the other channel.
I routed the wiring around the pcb to keep it out of the way and make it look somewhat neat, and went for the full test.
Sure enough, the headphones worked fine as normal. Then once I plugged in the ear buds the headphones went silent and the sound came out of the ear buds instead.