FYI: If you go to PBS.org, I believe you will be able to watch the full episode that you caught the end of, and the episode from the previous week. But as far as I know, they only post two episodes at a time, so I think you’re outta luck on the earlier ones until they air them again.
Correction: You can now only see the last episode:
http://www.pbs.org/wgbh/masterpiece/lewis/watch.html