Not sure if that Will entry relates to the Cuthbert who married an Elizabeth Cuthbertson. This is what I meant by there are a number of Cuthberts all around the same time and area

This is one family group with a Cuthbert for example
http://home.comcast.net/~derek2000/tree/gggg_gp/cuna1759.htm (mention of wills there)
There is an Old Parish Register entry for a marriage for a Cuthbert and an Elizabeth Cuthbertson on
22 October 1791 in Stewarton. Remember though, this is all before the start of official registration and parish entries are likely to be minimal in content.
Ian, if you, at this stage I would research whether there may be a monumental inscription to be found for James Ferguson and wife Elizabeth Nairn for possible clues. Also, possible grandchildren's names for them to see what if any clues you can get regarding possible grandparents' names.
There is nothing at the moment to give you clues as to possible names for James Ferguson and Elizabeth Nairn's parents other than the names they chose for their children (having said that, although we can't see a death for Elizabeth post 1855, there may be one for James Ferguson?).
Monica