This thread may be helpful:
http://www.yorkietalk.com/forums/tra...you-bring.html
I have never found Benadryl to be helpful / make them drowsy or more settled. I do have to give Acempromazine (sp?) to one of my dogs when they fly, it's a must for him. It's a tranquilizer and if you give it, you do want to try it beforehand to see how your dog reacts and make sure they react fine to it.
For this long of a flight, it's tough to give advice as I've only flown in the US. Just be sure and check on your kiddo often to make sure they're not cold and/or overheating.