Dave, the law is;
1. Tourist visa = British licence is all that is needed, as long as it covers the class of vehicle you are driving.
2. Residence permit = British licence translated into Turkish and notorised, and it covers the class of vehicle you are driving.
3. Turkish nationality - Turkish driving licence required.
If you are here on a residence permit, and you only hire a car infrequently, by the strict letter of the law 2. applies. If you are on a tourist visa then 1. applies. Its up to you how you apply it.