Hi Miles and welcome to the Club. My S type, which preceded the XK, and which has a lot of things underneath in common has "four jacking points on the floor of the car". Rather vague. My car came with a spare wheel as specified by the first owner. I take it that the XF might be similar. On my s type, there is a gap about a foot along the sill and at a quick feel it seems that there is some kind of support. Before trying to jack up the car I would be incline to get a suitable piece of wood at least an inch thick, 6" wide and about 18" long and if you can see if the jack will support the car. Hopefully an XF owner might be along to give you a little bit more certain advice. Regards, Peter.