Should have taken 10 minutes, but took over an hour.
The soft-close seat was not closing softly after almost 6 years of use, so I got a replacement. One of the nuts holding the old seat fixing undid fairly easily with the supplied plastic socket. The other one did not. Using a spanner just resulted in the 70mm-long threaded S/S rod turning as well, but the top end of the rod was hidden under the round metal seat fixing on the top of the pan, so I couldn't grip it. A squirt of WD40 on the nut didn't help.
I decided to use a mole wrench to hold the lower end of the rod while trying to turn the nut with the spanner. But I was aware that if I crushed the thread I would not be able to get the nut off the end. But whatever I used as a pad between the wrench teeth and the thread was not strong enough to prevent the rod turning with the nut. In the end I decided to use the wrench without padding, and if the thread was damaged saw off the rod with a Dremel (there was no room to use a hacksaw). I got the nut turning without the rod, but then noticed the rod was slightly out of true. Carefully straightening it allow me to get the nut off somewhat more easily; fortunately the thread hadn't been damaged by the mole wrench.
Of course, all this was done in a most awkward position, and with the knowledge that too much force in the wrong place could lead to an expensive replacement for the pan!