And what would that be?

A light application of proofide top and bottom.

You can tension your saddle with a special Brooks spanner you know?

That wouldn't help, and might make things worse. It doesn't sag
longitudinally, so there's so need to tension it. It sags only under
the sit bones - two impressions a couple of centimetres deep that would
be fine if the rest of saddle was also at the same level, but instead
remains high and hard.

That happened to me too. A light application of neatsfoot oil
recommended by Sheldon Brown almost ruined my B17 as well. 2 deep
impressions created by my sit bones raised the center ridge by a bit
too. What I did next was to use the spanner, tightened the leather a
bit and tilt the nose down a bit (not up). After riding about 1000 km
later on, a miraculous thing happened. The hard ridge that caused so
much pain on my rod is now softened enough that I can tilt the nose
back up again. It's now very comfy even riding on my drop bars
(usually this position will numb it) for hours on end.

However, I'm not repeating the same painful experience with my B17, so
I'm planning on getting the Titanico from Selle Anatomica for my new
road racing bike. This is essentially a B17 with a center cutout.

Perfect!

David.