Having considerable experience operating into Derry, I can only say that the FO did the right thing and went back to STN. Derry is a very short runway, only an ILS on one end (RWY 26) with NDB non-precision on RWY 08. Who was to say that R26 was in use? I have no idea what the weather was like there last night but it's normally blowing a gale across the runway and raining! Would you want to land at an airfield with performance limits in a windy and dark night single pilot in a B737-800? No thanks! The flight is very short from STN so a diversion back is no great length of time and the guy was still breathing, he was not about to die. (I know this for a fact). So why not go back? The FO is to be commended on his excellent work.