Quick question, is the Z4 equipped to fit a hardtop?
I think all UK Z4s are equipped as standard.
If you look at the front of the roof frame by the doors with the roof down, you'll see a rubber cover over the mount points for the roof. Peel those off and you will see where the HT locates. Can't recall if both sides have it, but one side at least has an electrical connector for the heated rear window.
The front locates into the top of the screen just like the soft top - it uses an allen key to latch. The same key locks the two mounts behind the doors.