GW Marshall is of interest to those researching Cumberbatch genealogy in that he wrote a book called "Collections for a Genealogical Account of the Family of Comberbach" in 1866.
A brief biography can be found in the "Dictionary of National Biography Supplement 1901-1911" pages 573 and 574. He was born in 1839 the son of a Birmingham Banker. His mother was born Eliza Henshaw Comberbach; hence his interest in the surname. GW Marshall trained in the legal profession but his main interest in life was genealogy. The above book was the first he wrote at the age of about 26. Over the years he wrote many other books and was involved in the publication of a number of journals as well.
In 1897 he joined the (English) College of Arms which is responsible for the production of coats of arms. He was originally "Rouge-Croix Pursuivant" but in 1904 became "York Herald".
In 1891 Marshall purchased the Sarnesfield Court estate in Herefordshire. He died at his London house Holmbush, Barnes in 1905 and was buried in Sarnesfield. He was married twice: in 1867 to Alice Ruth Hall and to Caroline Emily elder sister of his first wife (no date given). He had 6 sons and 2 daughters (no indication as by which wife).
Given that he was always interested in genealogy it seems likely that he would have collected more information about the Comberbach family between 1866 and his death in 1905. A long time ago I wrote to College of Arms to see if they had information but it appears that none of the deposits he left relate to Comberbach. It would be interesting to know if any of his descendants have any information but I have not pursued this line.
Sarnesfield is a village off the A480 11 miles NW of Hereford. I've never been. Whether any of his descendants live locally I don't know.