At the moment, it seems to be mainly Thomas Cook who are cancelling or changing flights. They tell me its down purely to lack of bookings. However, I can't really understand how they can make changes 5 months ahead as so many people book much later these days.

All I can suggest is that if you are going to Turkey this year, book early - if there are enough seats booked on a flight, then they won't cancel. However if you leave it late in the hopes of a cheaper flight, then you might be left without a flight at all.