Officers in the British Army traditionally enjoyed the services of a uniformed personal attendant known as a batman, who helped with general personal care, cleaning and maintaining the officer's uniform, and performing other menial tasks. A batman would often continue into civilian service with the officer. The system was phased out in the British Army after the end of the Second World War.
A recent Times article revealed that the batman system was maintained by the Indian Army long after WW2.
Stevens is talking about Operation Overlord, the D-Day Allied Landings on the coast of Normandy on 6 June 1944, which marked the beginning of the end of World War 2. Five beaches were chosen for the landings: American forces would land on Utah and Omaha, the Canadians on Juno, and the British on Gold and Sword.
The batman uses the term "posh geezer" which makes him a southerner and probably a Londoner. It's therefore more likely that he was attached to the British 3rd Infantry Division landing on Sword rather than the 50th Northumbrian Division on Gold.