I've finished this project. Today I soldered up the second board, tested and installed it.
The slots for the 3-pin connectors with their 3.5 mm pitch pins that didn't fit the Veroboard I made with a Dremel tool with a small, ball-shaped, diamond bit. This went much faster than the method I used on the first board, which was to carve the little slot with a pocket knife. The slots came a out little too wide, which made soldering more difficult. The solder would not naturally make a solder bridge between the pin and the track, because the gap was too wide. I solved this problem with a small, fiddly, U-shaped wire bridge, that I soldered on the tracks.
A lot of the work on this project went into making these connectors fit. I'd recommend buying the right things. I could also have taken wires from the board to chocolate blocks, but that's really a bit crude, isn't it?. And I had the screw connectors and I wanted to use them.
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.