revlac,
I check tonight, but I think it came from HES Simmons notes relating to Markin's Mill. He probably was refering to the early OS map, though I understand that there is some debate as to exactly when each sheet was mapped, I have a facsimile of the map at home, and will double check. There was definitely no mill in Snape mapped on the earlier 1783, though there was one about 1 mile SW at the Langham Road/Blaxhall Road T-junction.
So if HESS was correct both Snape mills would date from between 1783 and 1825. I believe Snape Street Mill dates from about 1798.
Windy_miller