Sir William Martyn of Athelhampton, near Dorchester, Dorset (c. 1446 – 14 January 1503) was Sheriff of London in 1484 and Lord Mayor of London in 1492,
More information on the Wikipedia page [1] and alsohere and here.
An image of his tombstone is here
For the clergyman who was a member of the School Board for London see William Martin.