Let him/her know there are only 3 grub screws supplied with the kit - not 4 as erroneously seen in instructional diagram No.5. The lower wall fixing point has a self-tapping screw with a wide head (supplied) that is inserted from the top.
The 3 grub screws supplied anchor the nipple connection and the high rise shower arm. I would suggest to the plumber that grub screw No.1 be inserted before the rail is installed.
Cheers.