Central locking issue

[Doneill]

Hi all.. new to the forum. My sister gifted her 2005 polo to my 17 year old daughter ( her Niece) to learn how to drive. Great wee car with full service history and no impact damage more to the point. My issue is that the central locking on the drivers door isn’t operating with the lock button on the door card nor with the 3rd party alarm that’s fitted (Laserline). The 3 other doors operate ok with the fob and the button. Ironically all 4 doors operate perfectly with the key!!! Where do I start the process of investigation. Drivers door?? Central locking module?? Or 3rd party alarm module?? Any thoughts would be greatly appreciated. I’m a stickler about things working properly.