His real name is (get ready for this) Percy James Patrick Kent-Smith. He was born in Scotland but raised in Ireland. His mother was Irish and his father, who died a month before he was born, was English. Here's his Wiki article.

I love DVD commentaries, especially cast ones. I don't remember if you've seen Five's last episode "The Caves of Androzani" but Peter Davison and Nicola Bryant are in the commentary along with the director or someone and the comments he makes during the regeneration scene are HILARIOUS.

And yep, Jamie's the companion. And Victoria as well. It's also, I believe, the first appearance of the Cybermats.