From the Stowe 1848 catalogue, page xxv:

A Doric Arch... In a secluded dell, with a short distance from this arch, is placed a small Marble Urn, inscibed with the letters A.E.C.B.&C. 1836," and dedicated by the present Duke of Buckingham in memory of his mother, the late Duchess of Buckingham and Chandos.

Does this memorial still exist?