We know that William Herrick was already a vampire during World War I, where he turned John Mitchell into one as well.
Here's what Wikipedia has about the character:
William Herrick (Jason Watkins) is a socially powerful vampire who is interested in the way Mitchell lives his life. They used to be close partners but grew apart as Mitchell decided to try and control his vampire urges. Herrick was the vampire who "recruited" Mitchell, during World War I. Herrick is the leader of the vampires based in Bristol, their headquarters being a funeral parlor called B Edwards. He works as a police sergeant. He is leading a vampire styled Final Solution, which sees the whole world either turned into vampires, or kept like cattle as food. Their motto is 'No one gets left behind'. [Watkins's portrayal of the character was designed to be "more down-to-earth" than that of his predecessor in the role.]
I took out the next paragraph - it's a spoiler.
So there could be many family trees out there in Toobworld that can be traced back to William Herrick. And every so often the tele-genetics of Toobworld could produce a carbon copy of the source for those lineages - such as:
Oswald Cooper in 'Lewis' - "The Great And The Good"
Havers in 'Holby Blue' - "Episode 2.11"
Murray in 'Hotel Babylon' - "Episode 3.5"
Desmond Marshall in 'The Last Detective' - "Once Upon A Time On The Westway"
Barr in 'Blue Murder' - "In Deep"
(This is a picture of Jason Watkins in "Wild Child", but it could have easily been of Oswald Cooper at least.)
But Herrick can't be the ancestor for Colin Merrick in a second season episode of 'Life On Mars'. I won't play Spoiler King here for those still not exposed to one of the greatest TV series ever. But those in the know understand why this is not an option.