Speaking of blasts from the Whovian past, according to some cut dialogue, flatmate!Harry is the grandson of Harry Sullivan -- who isn't the grandfather who got arrested for trying to steal bits off the Great Wall of China, but flatmate!Harry's other grandfather. Noice.

Oh, gosh, YES. I love that! I don't care if the dialogue was cut and he never appears again so it's never confirmed onscreen, this is my headcanon :-)