Hallo again folks,
sorry it took me so long to post a reply.
After taking encouraging measurments at my local dealer’s display bike I did the only reasonable thing to do and had them order me an SX which I used for a month now an which currently takes a break neatly stowed in my locker.
And here’s how it fits: as mentioned, my locker is 49.7 cm deep and the SX fills 49.5 cm after sliding the saddle fully forward. So there’s still a comfy 2mm gap between the saddle and my locker’s door 
.
By the way: the locker’s width isn’t problematic at all as it is 30 cm with a door 25 cm wide. I got rid of the standard folding pedals and installed quick-release pedals (MKS Promenade EZY) so my bike is 21 cm wide now.
Cheers!
Simon