Hi again Chris, in case this is still of interest (!), I found out a little about Agnes' husband. He was Colonel Frederick Wright Hands, a colonel in the 105th Light Infantry, and his daughter's husband, Simeon Harrison Hardy, was a captain in the 74th Highlanders. The colonel's son Robert was described as a "private gentleman" in the 1881 census, so presumably didn't follow his father into the army. So no family connection that I can find with the Royal Engineers, which puts paid to the intriguing idea that William was the Colonel's batman!