Matt,
Apologies for neglecting the other part of your question. The nuts on my half-dozen or so Brooks (B.17, yours might vary?) saddles average 13.34mm (13.28-13.48mm) across the flats. The "Official" Brooks spanners I have are all a poor fit, being either too large or too small, seemingly depending on die wear and plating thickness. A 13mm combination open-box spanner will *just* wedge across the flats, but poorly. A 14mm is too big. In SAE measurements, the average .0524in, just too large for a half-inch. A 9/16 fits poorly, but will turn.
Fortunately, the nut doesn't need turning often and doesn't (shouldn't!) require high force, so a slightly too-large wrench is probably the best alternative to use. 17/32in should be spot-on. A 5/16 British Standard Whitworth wrench may well work also.
Keep in mind, Swallow nut sizes are different from the rest.
Best,
Dan.