Send a message to the person who requested a photo of the grave monument of Elizabeth Mercy Nicholson with the following comment:
I am researching why in my possession (inherited from my great aunt, I think) I have a copy of the will of an Elizabeth Smith who died in 1877. Elizabeth Mercy Nicholson was her niece and administration of the will was granted to her. My ancestor, presumably, appears to have requested a copy of the will in 1881 from Somerset House and I have a copy of that letter too.