What I have gathered is that at the beginning of the story, Shelley was in Andy’s room in this flat, because he had asked her over nominally to study, but in actual fact to attempt to put the moves on her. Jack has found himself living in the same flat as Andy, and somehow, by means yet unknown, at the same time. Paolo, Donita, Jim, and Kirsten also live in this flat.

Shelley, however, appears to live in a different flat, with Della, who is currently making casserole. Shelley has now left her flat and come to Jack and Andy’s flat, presumably to see Jack, as I doubt that she’s here for another round of fending off Handsy Andy. This explains why she’s ringing the doorbell, and also the deeply confusing mystery of why she was putting on her coat on the last page.