About a month ago after a drive, I noticed my left headlight washer cover was missing. After some research on this site and other resources, it seems like it is a common problem.
The cover is not available separately so I purchased the assembly from astonmartinbits and installed the cover/nozzle piece of it today. Here are the steps I used to complete the swap-out.

This involves getting under the car, so please take all safety precautions.

1. Jacked up the left side of the car and secured it.
2. Removed left front wheel, wheel liner, and front undertray.
3. Removed Air Box.
I read on some sites that indicated the headlight needed to be removed, but I found that this was not necessary as there was full access to the assembly after removing the air box.
4. There is a clip that holds the cover/nozzle to the piston assembly. I removed it and separated the nozzle from the assembly.
5. Removed the two screws holding the piston assembly.
6. Separated the new cover/nozzle from the new piston assembly.
7. Moved the existing piston assembly forward and attached the nozzle/cover to it through the hole and moved back into place.
8. Installed the clip to secure the nozzle/cover to the piston assembly.
9. Reinstalled the two mounting screws for the piston assembly.
10. Installed air box and front undertray.
11. Installed wheel liner and wheel.
12 - Remove headlight was pump fuse - F26 in my car per the owners manual.

Here is a picture of the assembly after removing the air box.