Battery in Key for remote opening

[Titan-S]

Just changed the battery in the key (as can be seen in a number of Youtube Videos) successfully opened the key and carefully removed the battery without any damage, replaced it and went outside excitedly to see if it worked...and... NO.. it didn't make any difference...

Anyone else had this, I did see somewhere that they need programming by way of key in ignition, press button etc etc,,

Key starts the car fine, but frustrated here so any advice before I hand BMW a wad of notes