In the video I've linked to below, Ricky Gervais talks (after a few giggles) about the moment he came to understand what to write about.
There's a bit of contradiction there, in that his experience with the old lady's flat was not something he found particularly interesting, it wasn't something he was passionate about; however, it seems that perhaps writing about it is what made it interesting.
It's an approach reflected in his most successful show to date, The Office, which mined the day to day pretensions and conflicts in a very ordinary setting.