I had a quick browse through Chorleys Bomber Command volumes and could not find any details so far, If this gentleman survived the war and you are looking for his Operation Records (ORB's) I would suggest you you try the National Archives in Kew, England.
They can provide you with his details, at a cost. Regards, John
P.S. - Some other viewer from this site may know of him and be able to offer some detail, otherwise try the above.
Mr. George Denis Scott Koester, was survived, and become coffe productor in Senahú Altaverapaz Guatemala Central America. please see in www.ensenahu.com. att. Abraham Elias.
Koester was Navigator for J.C. " Monty" Thelwell at 109 Squadron and were both posted to 105 Squadron on July 12 1943, when 105 began using Oboe and they needed experienced crews on the Squadron. Koester and Thelwell were on the very first Oboe op to Essen in Dec.1942. Koester flew Oboe ops with 105 Squadron until being posted to the Air Ministry July 12 1944. Regards Dave Wallace