When does Secret Diary of a Call Girl return?

The series was written by Lucy Prebble, who is also the author of the plays The Sugar Syndrome and Enron. It stars Billie Piper as Belle, a high-end call girl in London. The series has been compared to Sex and the City by many critics, mainly due to its humorous approach to sex. It is based on the blog and books by the pseudonymous Belle de Jour.