Many reviewers and fans thought the same thing when GP came out, Judy. Plenty of people thought they were referring to the Hitchcock-directed silent movie and protested that it had been a big hit, not the flop Lady Trentham mentions in GP. I believe there's a reference to the confusion in Robert Altman's commentary track on the DVD. I confess that I didn't even know there was a movie called "The Lodger" when I saw GP for the first time. It was only after I heard about the mix up that I sought out both films.

"The Phantom Fiend" is the title they gave the movie they're speaking of in GP when it was released in North America. The problem stems from the fact that the second version of the movie, the talkie that Ivor Novello made in 1932, was (and is) also known by the title "The Lodger" in Great Britain.

I watched both movies, too. The original is a great thriller, a classic early Hitchcock film. The second, while less well made, is interesting for Novello's performance and for the fact that he added music to the story, so you get to see him at the piano. Fascinating for JN fans to see the man he was portraying in GP.

More about the second film here:
http://www.alostfilm.com/2011/10/lodger-1932.html