Eleventh Doctor - Series 1/Series 5/Series 31 

The new series (i.e. 2010 series) should be called Series 1, as the production codes are 1.1 (i.e. Series 1, Ep 1), 1.2, 1.3, etc 

From Doctor_Who_(2010_series)#Production:
"The March edition of Doctor Who Magazine, referring to this as Series Thirty-One, confirmed production codes in the range 1.1 to 1.13."

Also, the head writer Steven Moffat calls the series "Series 1".  (Talk to me baby! þ)