Well we've cracked it. I now know how the Marshes/Crosslands of the dictionary are related to my Croslands. George Cros(s)land, the father of Joshua Marsh's wife Mary, is the older brother of my great great grandmother Mary Crosland (later Barnes) - both being children of Dan and Mary Crosland originally of Hepworth. I now have quite a comprehensive file on that Crosland family in the early/mid nineteenth century if anyone wants to know more.
It may also be of interest to note that George Cros(s)land's wife Ruth was a Heywood and that Joshua Marsh's mother Hannah's maiden name was Mellor - hence the Mellor connection of the dictionary.
Many thanks for helping to make all this possible. Without the information on the dictionary I would almost certainly not have worked out why Joshua Marsh was the informant on my great great grandmother's death certificate.
R