Peter, how old was the William W. you found in 1841, and where? Ancestry has 14 William Watersons listed in the index but most are children, and none in Lonan or Douglas.
Brian Lawson's 1841 census index has listed 28 William Watterson/Watersons over the age of 20, with just one in Lonan.
[WATERSON William 30 Lon 1464/2 5 4]

- I have had a look for this one, and see it is the one to which you referred! The "De" is actually "Do" - ditto. Meaning Strand Quarter of Lonan (Lonan being written above.)

It is difficult to read the name of where he was farming. I think it is "Lhe Kerroo".

Sue