The 'Cadfael' mysteries are set at an actual abbey in Shrewsbury, which was founded late in the 11th Century. In 1147, the bones of St. Winifred were brought there from Wales to be housed, and that event served as the inspiration for the very first "Cadfael" book by Ellis Peters, "A Morbid Taste For Bones".
So Brother Cadfael, although a fictional character himself, is working in very real settings - in much the same way as we can say the same for Columbo in Los Angeles or Adrian Monk in San Francisco.
And as such, he has come into contact in the series with historical personages. In fact, right from the very beginning he had the occasion to meet King Stephen (in "One Corpse Too Many").
However, today's nugget of information concerns two regular members of the cast who turn out to be actual characters from the pages of History - Abbot Herribert and Prior Robert, who served in those positions at the Abbey at the time when Cadfael would have been there in 1138.
I'm not sure the actual Prior Robert would have been too pleased with his televersion however. He seems like he could have been the spiritual template for Major Frank Burns of 'M*A*S*H' - a thick-headed, wrong-headed bureaucrat always at odds with Cadfael.