I've got John's new anthology, in which the Swedish version of What kept you so long can be found.
In the English version in the collection Seize the Night, in page 489, you will find this passage:
I would have cheered, laughed and sung to the upbeat song playing on the radio.
Guess what song is mentioned in the Swedish text?
Flickan i hagen, the song that Oskar played to Eli in the novel!