Except if you want to fly at night other than when you are training in the US you will need the rating before you go. You are supposed to have it on your license (in this case it is more like 'have the restriction removed') in order to be able to use the privilege with your restricted FAA license.
When I did mine I took the log book and license into Gatwick and waited for them to check it out and issue the amendment. I think all they do is reprint the one page that mentions not flying at night. It's worth giving them a call to see if you can do this.
Where are you flying on October?