My key fob battery retention differs to those shown in the youtube videos.
Mine has a cage that suggests that i need to split the case further to access it.

Can anyone shed any light on how to do it ?

thanks

 

From the way it looks to me, the green collar just presses down and rotates clockwise and holds the battery in place. You might need a small screwdriver to push the green collar counterclockwise back and lift the collar.

You can try this carefully so you don't force or break anything.