Stu DIG-S Smith

Hi All. Recently bought a 2012 1.2 DIG-S Micra, but have noticed that the start stop feature never operates - (yes, i've enabled it with the press switch, and stopping, in neutral, handbrake on, feet off pedals :). This is after a mix of driving, not just short cold-engine conditions. Also, no load other than ignition. I have heard that start/stop wont operate if battery conditions are below a certain level? Battery is brand new. I have done some quick diagnostics on the battery. 12.42v with engine off, rises to 14.4v on starting, but as choke adjusts and tick over slow to normal idle, volts drop to 12.3v (guess this is because alternator is supposed to stop - as it reverts to operating in regenerative mode??) Turning on headlights and rear de-mister volts rises to 13.2v (alternator coming online?) Volts drops back to 12.08v when lights/de-mister turned off Does this sound like correct operation of alternator? If so, I am puzzled why start stop never operates. Any advice greatly appreciated.