Series 7. 

While the BBC has confirmed that the series will return into Spring, until it is officially confirmed, please don't put "4th May 2011", as mentioned by Robson Green on his twitter. Yes, he may be a well known actor of Northern Lights, Casualty, Wire in the Blood, and may have had a few hits, even a couple of #1s, with Jerome Flynn, but it's not confirmed officially. Just saying.