I'd done the wrong thing and connected the power to my circuit the wrong way around, burning out the transistor. (I didn't know that Vebo on the 2PC1518 transistor is a measly 5V).
I did some research on reverse polarity protection, and it seems that a diode in series with the power input would be good enough. There's enough voltage (12V), so the 0.7V forward drop is not going to have much of an effect.
So there it is: D2
Squeezed on to the Veroboard too:
It looks worse than it will be: the diode can go in at an angle, but I'm using the free version of VeeCAD, which can't show that.