I have had my 2011 XF 2.2d for 3 weeks now and the stop / start has never worked, any ideas

 

Wrong section, should be in the X250 section, but anyway you may not even have stop/start, as it was introduced on the 2012 XF.
Easiest way to check is the pull up the boot cover and have a look at the battery(s) next to the spare wheel. If you have stop/start there will be a smaller secondary battery behind the main battery, no secondary battery = no stop/start.
Later on, MY 2014 I think, JLR changed to a single/main battery which also ran the stop/start.
Count yourself lucky if you don't have stop/start, it's annoying and pointless.