Thanks to those who replied to my question - I have the sheet music from an old collection with Joe Loss pictured dated 1939 - but looking on Google as suggested it seems that Glenn Miller played it earlier - anyway, whichever it was its a cracking good piece.
Joe Loss 'covered' a lot of Glenn Miller's music, but yes, it was originally a Miller song.
My dad was a big Joe Loss fan and always used to say that he was the British answer to Glenn Miller. I would dispute that but I guess it's a matter of opinion. I agree though - great piece!