Quick update if anyone's following the progress...
Spoke to both Husky and a helpful tech at Nissan HPC Oxford who gave me a list of fuses/relays (74, 86, 72, 71, 78, 76) within the Intelligent Power Delivery Module (IPDM - fusebox to you and me) which could be responsible. To get to this you have to remove the battery cover, then the plastic around the battery cover, then the plastic around that. Eventually got to the box - all the fuses were fine, and none of the relays were labelled, not even numbered.
Both the bloke in Oxford and Husky have never come across the problem before which is the story of my life with this car, and there's no information anywhere that even mentions that the lights should have auto-levelling. As such, I have recently acquired a PH2/250mm screwdriver which fits down the little hole to the adjustment screws and I'm going to turn it until the man at the garage gives me an MOT, then I'm going to pretend the car never had the stupid, pointless system in the first place!
If anyone stumbles across a solution or a decent IPDM diagram in the future, do let me know!
Cheers