When i bought my XF in September last year, the Stop/Start wasn't working and they checked the Battery and it was 75% charged and seemingly the Battery has to be at least 85% for the Stop/Start to work.
I kept asking the Ford Garage to get this sorted, and i took the car in on the 9th of December and they charged the Battery over night and the Stop/Start still didn't work.
So they fitted a New Battery which has a Higher Rating 95 AH/ 950 A as the original was 80AH/ 800A and the Stop/Start is working as it should, and starts working within 2 or 3 miles of leaving my house.
Regards Tom.