Getting back to it: I quickly tested my circuit with an actual automotive relay connected to it and it worked!

I decided that it was the time to get this in a box. Drill a hole in the side and fitted a grommet, unsoldered all the wires off the switch and relay, passed them through the hole and resoldered all the connections with heatshrink:

I glued the PCB to the lid of the box with some hot glue, which I don't know if it was a bad idea, but the alternative was Araldite, which would be more permanent. Then using a faston crimp terminal, removed the red plastic sleeving and inserted it into the pole terminal of the relay, making it change over configuration as required. 

I could glue this contact in place as it doesn't have the tang to hold itself inside the relay holder like the other contacts, but its holding itself with its own retention well enough.

It is now ready to be installed behind my centre console in the landrover. A job for another day.