I noticed that Pam St. Clement's name didn't appear in the end credits of Barbara's final episode. Wonder why?
It's something to do with credits getting published beforehand, and they wanted to keep it quiet.
Oh please... Doctor Who successfully kept quiet Jenna Coleman's appearance in two episodes (2012's Asylum of the Daleks, five episodes before she was supposed to debut, and 2015's Heaven Sent, in which she only appeared as an imagination spot after supposedly dying in the previous episode). She was credited for both. How is Pam's appearance as Pat's ghost any different? Don't publish the credits in the Radio Times, fine, but she should've been credited on screen.
Maybe because she was a ghost or hallucination, she wasn't really there so didn't need to be credited
An actress appeared playing a role - she would be paid, so would normally be credited. Probably just an attempt not to confirm her appearance until the episode played.