It looks like a Sidcot suit. This was a one-piece overall-suit developed by the Australian aviator Sidney Cotton in the First World War. For (relative) warmth and convenience it was so successful that the Germans were not above using captured examples; even von Richthofen was said to be wearing one when he died. Cotton refused to profit financially from his wartime invention. Wikipedia confirms that the Sidcot-type suit continued to be used by the RAF into the 1950's, though by WW2 it may have had a different official designation, and I couldn't say what that was.
