Cedric Diggory’s dad Amos really has done it all. He popped up in Doctor Who as Plantagenet, spin-off series The Sarah Jane Adventures (as museum curator Lionel Harding) and An Adventure in Space and Time as Mervyn Pinfield, an Associate Producer of the series.
Hogwarts’ cantankerous caretaker Argus Filch has made not one, but two Who appearances. Well, technically. He popped up as Solomon in Dinosaurs on A Spaceship back in 2012 before playing the very first doctor, William Hartnell, in the Bafta award winning An Adventure in Space and Time. Oh, and he voiced one of the rather vicious vulture-esque Shansheeth in The Sarah Jane Adventures too.
His utterly terrifying Aragog left Harry Potter aghast, but Julian Glover made a rather more regal appearance in Doctor Who. He played Richard the Lionheart back in 1965 before reappearing as the alien Scaroth in 1979 serial City of Death.