This is very trivial, I know, but does anyone have a reason why Billie Piper came last in all the credits? I thought she would have came third after the Doctor and Donna as her return was bigged up the most so I was just surprised when watching it again that she was featured last! Even behind John Barrowman!

It's sort of a sign of respect. In the opening credits it was

David Tennant
Catherine Tate

(as per the rest of the series)

Freema Agyeman
John Barrowman

(previously main characters returning for the episodes)

With Elisabeth Sladen

(not a main character in the revived series, but getting her status as a legend recognised with the 'with')

And Billie Piper

(By being last, but with an 'And' it almost elevates Billie's status to the same level as Catherine Tate's)

A lot of television series do this - Anthony Stewart Head was credited with the 'And' as was Alyson Hannigan in episodes of Buffy the Vampire Slayer, being last in this way highlights the importance of the character AND the actor to the show.

I'm assuming the end credits merely just reflected the order of the opening credits