Hi Graham,
You do not say who you are flying with, i flew Stansted - Dalaman via Istanbul with Pegasus, the following may be different if you are flying with another airline.
1. No need to collect baggage at Istanbul, it will have a transit label attached to it at Stansted.
2. You will not need to check in again, you will be given a boarding card for your connecting flight at Stansted. You will pass through passport control at Istanbul and if you need a new visa you will need to get it at Istanbul.
3. The plane will go to it's stand at the domestic terminal and you will then be bussed to the international terminal where you will collect your luggage.