Re: Fires of Pompeii currently free on iTunes
If you did try to retcon Capaldi's appearance as significant, you're going to be busy, what with two companions also hanging around there too (as well as Karen Gillan, Capaldi's 'wife' Tracey Childs plays the 7th Doctor's alternate universe Nazi scientist companion Klein in the Big Finish CDs).
"Some days are better than others. They say that where I come from."
"Loudly, I imagine, on the day you left."
(Blake's 7 - Rumours of Death)