Hi, got an XE Jag. Set for the ignition switch off mode when stopped but it doesn't turn off except very occasionally. Does it need adjusting or does it need repair? Still under warranty but hope it's a simple fix. Thanks. David 

The stop/start system has many things that effect its operation, temperature, electrical load and of course, battery condition. First place to look will always be the state of charge within the battery.

I found my Audi stop start hardly worked during winter with the heaters on unless you had a good run to warm the car. I have only had the XF a couple of days but over 100 miles it’s only turned the engine of once.  Like @Jimbov8 mentioned it will not switch the engine off if the battery is low, your not at temperature in the car, if heated seats are switched on where it needs more power etc.

I looked into this subject in 2015 and the parameters mentioned (and in the handbook) affect it more in winter. JLR did a review and altered some computer settings to get less problems. In the end I bought a new JLR 90Ah battery which the dealer fitted and it's worked perfectly ever since. Whether they applied the update as well I don't know. I suggest charging the battery either with the under bonnet terminals or in the boot. DO NOT connect the -ve directly but to a stud in the boot floor. There is no need to remove the battery or undo it. The +ve can go directly to the +ve on the battery. A CTek or similar charger is the best type as it monitors the battery charging throughout.