Right, I’ve just got in.
Tried jiggling, rocking, moving the switch with my foot on the clutch, wheels straight, in 4H in 4L in 2H, and combinations thereof. No good.
Ok. A friend said take out the fuse. I looked for it. Couldn’t find.
Rang the RAC.
RAC came. Disconnected the batteries. No good. Took the switch out. No good. He left, saying he would send a recovery truck.
Then I remembered I’m in the Nissan4x4OwnersClub. I texted the chairman. The actuator was stuck and I was to drive round in very tight circles. It worked, almost instantly!
BTW, the going backwards thing is to turn your hubs off if you’ve got automatic hubs. I haven’t, mine are manual, the autos broke and Nissan wanted over £300 plus VAT PER SIDE, so I got manual ones.
Thank you all for your input, as always.