Dear CK,
Yes, thanks for reminding me to check the books (and photo galleries). Indeed sub-adult Heuglin's Gull types can not be ruled out very well. The key features are the wing pattern, bill shape, and the tail pattern. I think the wing pattern fit both, the bill shape fit Relict better, but the tail seems to fit Heuglin types better. I believe overall this bird fits Relict Gull better, but would welcome other suggestions and discussions.
HF Cheung