This is an extract from volume two of several volumes of a publication:
Royal Commission on Historical Monuments (England)
An Inventory of the Historical Monuments in Buckinghamshire
The entire contents is available on the Historic England web site. If you visit that site and search for Great Brickhill you will get access to a mass of information.
The document listed here is a copy for original manuscript and dates to the period. The entry for Great Brickhill starts on page 2 near the Bottom.