Despite my best efforts, I couldn't get a PCB made at my makerspace. However, I asked one of the guys in the electronics workshop at my university and he was very happy to help out. And this is what he produced:

So I fired up the soldering iron and put it all together:

I managed to program the Arduino Pro Mini with instructions from from here. Really very straight forward. Once programmed, I tried it out on my small gauge:

Worked first time! Well chuffed.