I just went looking for my son's copy of Night Watch - he keeps all of his Pratchetts in a totebag which he lugs between my house and his dad's. Apparently this one went to school with him today! Anyway, I was going to look up a quote, which I will have to paraphrase:
Sam Vimes thinks to himself about the song something along the lines of: It's the perfect soldier's song, being both sentimental and ribald.