This is priceless. So I eventually got an email from Erisin "Support" this morning. Apparently I just leave the 2nd wire dangling, and it will still work:
"That is ok to connect one brown wire to your steering wheel controls. It doesn't matter, when there are 2 steering wheel control cables then connect 2, when there is only 1 wire, then only connect 1. That is no problem, don't worry!"
So that's that sorted then
When the rain stops (predicted for some time in 2037), I'll try and connect everything up, finding an earth point for the 2nd wire as suggested