(episode #3.19), Will and Jack go to the opening of "The Atom Bar". Adam Barr was a writer and producer of the show at that time.
We find out Karen's age in this episode. Jack accidentally blurts it out. She is 42 as of this episode.
Nathan's apartment is 12C, three floors above Will's. It is the first apartment the crew made that actually looks almost like Will's. Previous apartments that shared the same number were studio apartments with missing space.
The final shot over the credits is confined to one room. So we don't get to see Will dump Karen's Mother-In-Law on the couch.