Reported on another site that flights were purchased for a Pegasus flight, through another agent, think last minute, and the girl was told on arrival at Istanbul that she had to check in again. As the length between flights was short, she couldn't so another seat on a later flight had to be bought. Some offers on sky scanner say self transfer, it is assumed that it means this. So if you do book through another agency, make sure you have enough time between flights to collect luggage and check in again!
Re Turkish Airlines luggage allowance, I meant that if you fly from Dalaman to London and back to Dalaman, luggage allowance is 20kg for each flight, if you fly London to Dalaman and back to London, allowance is 23kg for each flight, booked directly with Turkish Airlines.