Hi Roo-power,
Opinions seem to be concurring as to source of subject used in the TV episode - The Grinning Man. Please see the 2 refs cited below:
Re: The Grinning Man
Well the picture in the Grinning Man is from a picture by Hieronymus Bosch and is called "La salita al Calvario" and I think it was filmed in Knebworth House, Knebworth, Hertfordshire.
http://www.castelmeteo.it/arte/Hieronymus%20Bo sch/pages/Hieronymus%20Bosch%20-%20La%20salita %20al%20Calvario.html
It appears to come from this painting from Bosch, Christ Carrying the Cross:
http://www.ibiblio.org/wm/paint/auth/bosch/car rying/carrying.jpg
In the bottom right is a face similar to the one from the painting. It is facing the wrong way and the nose is slightly different; also the eye in JC is facing outwards. Furthermore apparently tobacco wouldn't have reached western Europe until 80 years after Bosch's death. All this said, it definitely appears to be based on this face; notice the hood or hat is identical, as roughly the mouth and jaw are.
Obviously the programme makers have indulged in some clever cgi editing and one day the author accredited to this may be revealed.
Until then thanks for your help.
Cheers