I'd double check to see if Thomas Cook have an online seat reservation system for your flight.
Only today I found the "manage your flights" section on Virgin Atlantic, entered my booking reference and obtained seat map of the aeroplane for both outbound and return. I'm quite glad of this as most of the seats have already been booked... I've obtained the *last* window seat left and that is 62A - three rows from the back of the 744. I've got 56K on my return.
If Thomas Cook does have an online reservation seat reservation system then be sure to use it and you should be okay to turn up around 2 hours before with the knowledge that you have your seat assignment in place already.
Have a great time in Calgary.