There is an interview with Joseph and Juliet Cowan in issue 410 of the Doctor Who Magazine (available tomorrow). Thanks to Sarah Jane TV for posting details.

 

Moving on, does Joseph think that Alan and daughter Maria's adventures have a shelf life beyond the confines of Bannerman Road, and will they miss the presence of Sarah Jane Smith in their lives? "Maria will, certainly," Joseph responds to the question, "I think, even though Alan is now in on it all, and he gets a kick out of it, he's terrified every time there's an adventure. Children have no fear, and Maria definitely doesn't seem to have any fear - but every time, Alan is like, 'We are going to die this time...' So, I think he's quite relieved to be leaving it all behind, in a way. But as we'll see, they carry on in America, he and Maria. There's going to be an HBO spin-off!" He jests. We think! "We would have stayed for life," Joe Millson adds, "And we'll always come back if we have the chance."