I wouldn't say it has moved past its prime at all. There was a series about 1.5 years ago (approx.) and it was hilarious. Saffy's had a baby and Edina is NOT having it. Until she finds out it's a mixed race baby and refers to it as "the must-have baby of the season. The Chanel of babies!"
It was 6 episodes, I believe (standard for a British "season")
Jennifer Saunders is notorious about delivering scripts late, and she's said she's done writing the show, but she's said that before. So let's hope for at least 1 more series.
http://preppylife.tumblr.com/