Sorted! Referring to the third picture I posted, I first slackened off the two Allen screws seen at the base of the handlebar clamp thingy, expecting to be able to pull that and the bars off upwards. But no: it didn't want to let me.
I then realised that what I had assumed was a plastic cover (i.e. the part labelled VP-Z051AD) over a threaded stem locknut is in fact the means of adjustment. It's alloy and it's threaded. So by gripping and turning that in increments clockwise together with the bars, holding it in place, and then twisting the bars back to where they were, I started taking up the slack. Repeated three times, Allen bolts tightened, job done!
Weird, but effective.