Andrew, I've just got started on this project and came acroos your old message. I'm not sure how far you have got with all this. I have some Private Notes and Records on the Sellon family passed down from my great aunt. These were prepared some time around the 1860's by John Scudamore Sellon, a great grandson of The Rev William Sellon and Sarah Littlehales. You have listed his father as Jean Sellon, however, from the notes of the family tree Rev William's father was Rev Walter Sellon, whose father was William Sellon. His father is unnamed, but this gentleman and his brother Jean, escaped from France after the Revocation of the Edict of Nantes in 1685. The notes show no further back but indicate that the Sellon or de Sellon family came from Nimes in the South of France. I have been trying to trace this back further. Jean's side of the family went to Geneva, and his daughter married Sir Charles Tyrell, and his ancestry I have traced back to 954 through the kings of England and Scotland. If you wish to see the family tree go to www.gencircles.com/ and check out the Sellon family surname Henry/Greig/Sellon family.
Hope this is of use. Please let me know if you belive that my info is incorrect, but the notes are in the form of a published booklet. I can send you a copy if you wish.