My 55TDi has it. Does it irrespective of the Start/Stop setting and the Aircon can be on, however I generally have my Aircon on Eco mode not full mode and most of the time I drive in Economy mode.
Little battery symbol on the end of the MPG display needs to be green.
I've noticed it mostly on the motorway when cruise control is on and the RPM drops to zero and the car speed can drop by up to 2MPH before it restarts the engine. It will also kick in if going down a hill on the m-way.
If not on cruise then you need to be off the throttle for a few seconds for it to kick in, again normally happens going down hill.
Maybe it's a 4pot / 6pot difference??