Okay, so after watching the movie again, I did a little research to try to find out when Jeremy is playing the piano for certain.
Here's what I found:
This is Jeremy playing during the first cocktail hour--at 28 minutes:1 second in.
This is the same time frame, only bigger and cropped. We see Jeremy playing very briefly and then we don't see his hands on the piano for the rest of the scene. I'm not sure who played this piece. It could be his brother Christopher's playing that we hear, but, if so, the music would have to have been added later because, well, we can
see Jeremy and his hands.
These are
someone's hands playing at the beginning of the 5-song interlude where Ivor entertains while the murder is committed. For about 6 seconds (from 1 hour:10 mins:10 seconds to 1:10:16) we see hands playing in close-up. The camera doesn't pan out to show who it is, so they could very well be Christopher's hands. I took the above 3 screencaps at 1:10:11, 1:10:13, and 1:10:16.
Now these next screencaps show Jeremy's hands for sure. They're taken when Ivor is playing "Keep the Home Fires Burning" the night after the murder. JN is listed in the closing credits as performing this piece.
This is from the 1:47:43 mark.
And this one's from 1:47:44.
What do you think? Are these all Jeremy's hands? Or is it Christopher playing in the middle three caps? (I apologize for the caps not being crystal clear; I had a choice between clearer small caps like the first one, or larger but less clear caps. The larger ones seemed to give the most detail. Plus, some of the blurring comes from the movement of the hands. Hard to catch them being still!
)