I also have a MY13 2.2d XF which i acquired about 18 months ago. When I got it the Stop/Start worked fine but a few months ago it stopped working. I have Ctek smart battery charger which on connecting to the Main Battery advised that it was either "heavily Sulphated", "could not be charged" or "could not hold a charge". The car was performing fine and started ok but as the battery was obviously the problem with Stop/Start. I ordered an Exide Replacement Battery from Tayna Batteries on line for £136 which was delivered next day, I fitted it and Stop/Start has worked flawlessly ever since.