Reaney and Birch
The derivation of the village name has many theories. It is generally thought that the Walter part is due to a quote by P.H.Reaney to Walter de Tey. The Belchamp part has many theories, from the French (Norman) through Anglo-Saxon and the description of an topographical feature.
Reaney quoting Walter de Gray Birch
I can find no such reference in P.H. Reaney's "The Place Names of Essex".
I cannot locate any reference to the 904 date in the copy of Cartularium Saxonicum by Walter de Gray Birch that I can find.
The links in the text below are to my research:
The village history from the Village Hall website
The 904 date:
Reaney does not "cite" Birch with repect to Belchamp Walter, he was refering to to the lost Old English word "Blyc" .
This is an example of misquoting the original. In addtion, the reply from Essex County Council, Hertiage Services quotes the date 940, not 904.