This fault is remarkably common in the X100 models. It's caused by a small design fault in the sizing of the drive cable assembly whereby the outer casing of the drive cable is a little too long allowing the inner cable to disengage from the mechanism.
Jaguar issued this TSB explaining in detail how to fix it
http://www.jagrepair.com/images/TSB/TSB ... rative.pdf
I had to do this for both my seats. I would suggest undoing and re-doing the zip/clip arrangement on the backrest cover is a hard earned knack and new hog rings are not really required. The only thing that makes this job difficult is trying to carry it out with the seat installed. Removal of the seats will save you both time and your sanity
NB the precaution of disconnecting the battery before unplugging any connectors under the seat, especially wrt the airbags in some models
If putting this job out to a garage, note the time allowed for the job by Jaguar - 0.4hr. Seems a bit optimistic to me but it should be helpful when negotiating the cost!