Jeanette13 - With regard to the diary of Donald Robert Clarke that you have transcribed, may I please ask if you have noticed anywhere in it a reference to a Marion Manson of 17 Dalrymple Place, Edinburgh who passed away in 1839?
This is very much a long shot, but I do know, from her intestate settlement document, that she was the sister of Barbara Manson, buried in Arnaboll, and she was possibly born in Thurso to an Alexander. Knowing that Donald Robert Clarke was in Edinburgh in the 1830's, if she was in any way related to him it seems possible that he might have visited her.
As I said, very much a long shot, but I am struggling to unravel the Manson's from Thurso and anything is worth trying!