Thank you to sillgen and hepburn.
I did check the St Margaret's website. It does have a list of burials, but generally only says "person X buried on date Y" sometimes with an age. I looked at a very dim microfilm of the parish register about 15 years ago, which has the same sort of sparse information. Fasham Nairn left money in his will to the clerk and sexton of the parish for the upkeep and regular repainting of the tomb. Is it likely there would be any parish accounts surviving that would record such payments?
An interesting sidelight - Fasham leaves a legacy to his cousin Charles Nairn Hastie whose father was HEPBURN Hastie. Any connection?