From the Stowe 1848 Sale Catalogue:

In the Lake [the Alder River] are several small islands, upon one of which is a monument to the memory of Captain Cook. Upon another, there stood a small stone altar, which had originally been erected by Louis XVIII, at Gosfield, in Essex, a house belonging to George Marquis of Buckingham, which that nobleman gave up to the illustrious members of the Royal Family of France, during the period of the French Revolution. In testimony of his grateful sense of the hospitable and generous assistance which he has received from the Marquis of Buckingham, His Majesty caused a Latin inscription, written by himself (of which the following is a translation), to be placed on one face of the altar:

This place by the generous magnificence of
George Marquis of Buckingham, and of Mary Nugent, his Marchioness,
Was inhabited, upon their first arrival in England, by
Louis XVIII, King of France and Navarre, and his nephew, Louis Anthony of Angoulême,
And afterwards by their illustrious Consorts,
Maria Josepha Ludovica of Savoy, Queen of France and Navarre; and Maria Theresa of France, Duchess of Angoulême:
The Elms around this spot were planted in their own hands, as a memorial to their unceasing Gratitude.
A.D. 1809.

The original MS. of this inscription, in the King’s handwriting, is still extant in the Stowe Library. When Gosfield was sold by the late Duke of Buckingham [in 1825], he moved the Memorial to Stowe, and inscribed on the opposite face of the Altar the following explanatory inscription:—

Louis the King having been restored to the throne of France; and Peace, by the blessing of God, to Europe;
Richard Duke of Buckingham and Chandos
Removed from Gosfield this monument of the munificent and splendid hospitality of his Parents,
And placed it amidst these scenes, surrounded by the Memorials of his Ancestors.
A.D. 1825.