Door won't unlock from outside until unlocked from inside?!

Hi

I have searched for the answer and have gained some bits of useful info but I'm a bit stuck!

The passenger door unlocks with the central locking but can only open if using the inside handle first. So once unlocked via central locking, i can't open it from the outside handle, only the inside handle - once done, the outside handle functions normally and opens the door.

I have taken it apart and checked the physical cable mechanisms which seem fine (presumed they would as both handles do work), and I can hear the central locking motor going if i put my ear to the passenger door, when the car is unlocked with a key in the drivers door.

I haven't tested the contacts with a multimeter as again i've done research but not 100% sure where to place the tester points.

Does anyone have any idea about this please? I read that it might be the door actuator but although i have taken the door panel off and have had a good look behind the foam panelling, I don't know where it is - is it basically the main lock unit itself? I don't really want to go spending money on it unless I am fairly sure that's what i need to replace!!

Thanks in advance