Well, untill the gear box is removed you won't know for sure.
Several posibilities.....
If your transmission/gearbox isnt making any scary noises I think its probably fine.

Most likely one of the following...
If these machines are fitted with a dual mass flywheel, the springs may have failed and the pressure plate part of the flywheel may have become partially engaged with the clutch disk..... if this was this dire, you would possibly notice a vibration with the flywheel running out of true & eccentrically....

MOST LIKELY issue is a failed slave clutch release. I m guessing that the XF has a hydraulic slave realease breaing and if so, a seal failure will cause this.
Its possible for the seal to not leak flud, but to draw air similar to the current FORD Focus & Mondeo's.
If you open seperate the engine and box, and you have a dual mass flywheel, change it as well as the clutch disk and the release bearing mechanism as a whole package.
You will usually get a guarantee on the parts from a garage for the whole package for miles or months after.

I doubt its a g'Box issues..... sounds like clutch release issues to me.

Either way, what ever sub part is at fault, if you've got it in bits, change the lot.

Regards
Grant B

feel free to ring if you want to mail me privately for a number.
Ive never seen the insides of the XF clutch mechanism, but it does sound like this is most likely