Hi Rory, and welcome to the Club!
I had the XF3.0D S five years ago, but sold it when I retired since I recognised that my mileage was going to make me worry about the DPF on the diesel. If most of your journeys are fairly long, and at 50mph or more, that worry may not be applicable to you.
Yes, my XK is thirsty, but I have no regrets at all.
Whichever you decide on, make sure that the one you are looking it has a full service history, is immaculate, and that everything on it is in full working order.