Hi Richard,
My XF-S (2011, bought a year ago) arrived with Chinese 'Triangle' branded tyres, which were truly 'interesting' in the wet.
Quickly swapped for a set of Pirelli P-Zeros (255/35-20), car was transformed!
Good luck with your search, mine were roughly £200 a corner at Quik-Fit, plus a few £££s for tracking adjustment. Next job is to get the wheels refurbished, but the plastic 'blades (5 needed per wheel) seem to be incredibly expensive!
Cheers,
Jon