PV Electronics were kind enough to sell me an HV Module to power the Nixies, which is not yet officially for sale on their web store.

NOTE: Nixies require about 170 volts to 'strike' the tube and make the [I assume] Neon gas glow. You can't just put 5 or 12 volts in and expect things to happen :)

I subsequently realised that I needed to find out how on earth this thing should be wired. After a bit of digging around, I found the answers I needed in a Schematic included at the end of the 'Spectrum 18' Assembly Instructions on the PV Electronics site.

From this diagram, I could see that the input power needed 12 volts, and that pin 4 (titled 'Radj' on the PCB silkscreen) needed a fixed resistor [6.8k] and a variable resistor [1k] to trim the HV output. After a quick hunt around the 'net, I also learned that 'SHDN' was short for 'Shutdown'.