The College of Arms, in London, near St. Paul's Cathedral, has various documents related to the family history, as well as the master copy of the Marriott Dodington Coat of Arms. Some of the older parchment documents are from the 17th century. The documents on file include the originals of Acts of Parliament which were needed to change surnames, as, for example, when a husband would adopt his wife's name. (this is how the Marriotts and Dodingtons came together, when there was no male Dodington heir.)
The official records for my section of our family have not been updated since the middle of the 19th century, probably due to the cost involved. Updating just my branch of the family was estimated to cost around $800: this involves careful handwriting of the various names and births and deaths.
A "Pedigree of Marriott-Dodington of Horsington", apparently done by the College of Arms, exists and I have a copy. The information from it has been folded into the family tree on this site. The references at the bottom are in part:
"extracted from the records of the Heralds' College, London, marked "D 27" p.38, and "Norfolk X." p.55, and examined therewith the 24th day of November 1863 by Us, E. S. Dendy, Chester Herald, G.W. Collen, Porticullis Pursuivant of Arms."
"extracted from the Heralds' Visitation of Somersetshire, An 1623, marked "2C.22", p.321; remaining in the Heralds' College, London"