I've done it twice myself, although both times because the pin broke. Once was 40+ years ago, the other much more recently.
The first time was a real struggle, but I managed it by using a long screwdriver as a lever. I may have had to use a vice or something to hold the saddle still as well (probably on the seat pin or similar). The more recent one just slipped into place no problem. Both worked fine afterwards.
You can't get some mole grips or similar onto the bolt to hold it while you shift the nut? And you are using a proper Brooks spanner, which will grip the nut better than a regular one?