How to remove tap valve within shroud (Franke ascona mixer tap)
Have you tried an open-end wrench?

Such as this:

https://en.m.wikipedia.org/wiki/Wrench#/media/File%3A2008-04-14_Chrome-Vanadium_Wrenches.jpg

It won't fit flat onto the nut but it looks like you could get a good grip on it at a shallow angle. The end of a spanner (adjustable wrench) is quite a bit thicker and harder to maneuver in tight spaces.

Otherwise, you would need a specialty tool to remove it.

Good luck!