It's difficult to know what to suggest this far down the line. The last three IBers that I knew personally and had an interest in the songs as well, (Tommy Chilvers, Jack Longstaff and Maurie Levitas) are now all dead. I never heard any of them refer to the song. Indeed I seriously doubt if there is anyone at all from the 15th battalion left alive. Have you tried the International Brigade Memorial Trust at 37 Reginald Road, Forest Gate, London E7 9HS. Tel: 020 8555 6674? They might have something in their archive, if the song was contemporary. But it may of course have simply been composed for the show you heard it on.