Thanks Joebhoy and Spanish Eyes. Grateful for your suggestions.
The site you recommended (Spanish Eyes) contains the names of Thomas and David Urquhart who were sentenced to death for stealing from the mail in 1797. I would love to show that these two were related to my wife but it seems unlikely !!.
From the very few Urquharts who appear in the Orkney records it is difficult to make sense of Thomas Matherson's book which says that Thomas and John Urquhart were a pair of pipers who came to Shetland from Orkney in the mid to late 1700's.
I'll keep looking. Your help is appreciated.