Is present perfect even possible for this sentence in English? My feeling is that if a time in the past is specified with 'left', and the action is a one-off - we always use simple past i.e. 'he left an hour ago'
I'm sure that's right. You could say "he went an hour ago" or "he had gone an hour ago" (very different meaning, of course). In German, "er ist vor einer Stunde gegangen" translates as "he went an hour ago". I wonder if Danish is the same.