We were definitely supposed to think they liked each other. When he goes back to her flat after their earphones session and her roomie ended up being home, when they locked eyes (can't remember the exact scene)... They were definitely attracted to each other. BUT, like real life, sometimes these attractions don't lead to anything. It was for the best that he went back to his wife and she was a single.
In our house, we all agreed that it was the only time that we were likely to say "Thank goodness James Corden appeared" (not fans). We were all worried that KK and MR were going to get together, and it didn't seem right.
There were subtle hints that he still really loved his wife and that she still had feelings for him, so we were quite happy with the ending.
To the OP: John Carney's films all seem to have endings that aren't quite satisfying, seeming to deliberately avoid a "normal" conclusion. Maybe this is why people weren't happy.
No tears please, it's a waste of good suffering.