steve
you should have posted this question in the thos cook airlines thread in the flights forum, this forum is for tour operator questions... as it is i believe the answer is your too early as next years schedule's will not be finalised yet, and i think you can only do this 90 days before your flight, but im sure the experts on the flights forum will confirm

wizard

Wizard is correct it's too early to book YSYC for next May. You will have paid the minimum to book this option which allows you to pick your seats a week before your flight. You also have the option to pay extra for priority choice which let's you choose your seats around 90 days before. This info should be on your invoice.

Also Thomas Cooks are changing their flight numbers for next year. At the moment they have a K on the outbound and and L on the inbound but they are getting rid of those as you can see with your flight numbers.
But as Wizard and shirley have said, it is too early anyway. 98 days prior to your flight is the earliest if you pay extra for priority service at £6 per person per flight.