"we’ve done six extra episodes last year"

TV SARAH-JANE.tv has the transcript from a Q&A with Anjli Mohindra who plays Rani in The Sarah Jane Adventures from the weekend's Manchester Comic Con. Interestingly she fairly ambiguous as to whether the show will carry on or not:

"What's happening with the future of the series?
We’re not actually sure, we’ve done six extra episodes last year when we filmed series four, so we’d be able to keep them in the bank and then film another six and put them all out this year in a series. Obviously, we’re just gonna stick with those six, I think, and just send them out in autumn. Err, we’ll see, I’m not sure to be honest, but there’ll definitely be another six in autumn."

I wouldn't be to averse to a spin-off of a spin-off with just these characters, a kind of sci-fi Famous Five affair.  With K9 in Timmy's role obviously. Script editor Gary Russell did after all play Dick in the 70s tv series.