The best intentions ...
Solved the mystery of the derailleur cable, all for one small but critical detail. The Brompton instructions clearly state it, and I thought I had one, but if you don't have a 1,5mm hex key you can't release the cable from the lever, and without that, you can't remove the cable at all.
In my case that involved a 50km hike to the nearest DIY store to buy one, after which restaurant duties called, so will have to finish the job tomorrow. Ah well.