Hi Claire_Bear
Dont stress too much about the flight... We flew to Paris with an 18month toddler last year this time, luckily also on a night flight. By the grace of God the flight going there wasnt full and we scored a "free" seat for her - we sat three in a row and the toddler slept in the middle of us... that was a blessing indeed! the lull of the plane, the hum of the engines and the darkness all contributed to her sleeping so well, we didnt need any medication for her. At the recommendation of the paed we just had Calpol in just in case, and Rescue Remedy...
On the flight back we flew KLM and didnt score a free seat! If you see this happening at check in - ask for a seat by the bulkhead. There is extra leg room there and with extra blankets we made a bed on the floor for my LG and she slept a good 8hours of the flight... and this was during the day! Again we didnt need to give any meds... Because it was a day flight when she was awake we let her roam around the plane to burn off any excess energy she may have had. But again, being on the floor she hum of the plane kept her in a deep sleep!
Hope this helps, and all the best!
Also, pack enough toys in... In seperate individually wrapped gift - that way you get maximum exposure out of each toy... Colouring books definitely help, and stories to read!