Ah, actually, thanks for checking the thread, but it turns out I'm good.
In a minor miracle of urban life, I just drove the few kms to the traffic lights where I thought I remembered a bit of a clunk happening at the back of the bike [last Wednesday!], and found the missing piece, a bit the worse for wear, in the gutter, a few metres from where I stopped the car.
Problem solved. Loctite on the job this time.