So, at this point, five of Merlin
's seven main cast members have appeared in Doctor Who
Richard Wilson (Gaius) was Dr. Constantine in "The Empty Child"/"The Doctor Dances."
Angel Coulby (Gwen) was Katherine, Reinette's handmaiden, in "The Girl in the Fireplace."
Anthony Head (Uther) was Mr. Finch in "School Reunion."
Colin Morgan (Merlin) was Jethro in "Midnight."
Finally, John Hurt (the Great Dragon) was the Doctor in "The Name of the Doctor."
Unless I've overlooked them, we only need Bradley James and Katie McGrath to appear to complete the set.