Usually, but not always, when leather saddles creak, it's the nose piece. Either the tension bolt and nut, or where the nose is braced onto the rails. The other common place for squeaking is where the rails sit in the post. I also had an old Brooks Pro develop creak where the rear support sat on the rails (at 30,000+ miles) and eventually the brace cracked there.

Next time, don't get frustrated, and use some thinned oil (1 part oil, 5-10 parts mineral spirits) on one suspected place at a time, until you isolate the spot where it made the difference.