Thanks for all the comments, in the end I went with the SU-4 mode on my Nikon SB-900, and putting the X-T1 flash in commander mode, worked flawlessly!
The Cactus V6 also looks like a solid recommendation, for the Yongnuo it's a bit troubling there are people for whom it doesn't work, hope we find out if it's user error or a problem with Yongnuo!