I came across a mention of a Philip de Crespigny being a prisoner of Napoleon Bonaparte in the 1889 book Englishmen in the French Revolution:
Philip Champion de Crespigny, brother of the first baronet—he was married at the Danish Embassy, Paris, in 1809, his bride having apparently gone over to share his detention—escaped from St. Germain in May 1811. He lived to be eighty-six, dying in 1851. ( From Alger, John Goldworth & Robarts – University of Toronto (1889). Englishmen in the French Revolution. London S. Low, Marston, Searle & Rivington Page:Englishmen in the French Revolution.djvu/295. (2011, April 1). In Wikisource, . Retrieved 23:10, August 31, 2013, from http://en.wikisource.org/w/index.php?title=Page:Englishmen_in_the_French_Revolution.djvu/295&oldid=2641334 )
The first baronet was Claude Champion de Crespigny (1734 – 1818). His siblings, the children of Philip Champion de Crespigny (1701 – 1765) and his wife Anne nee Fonnereau (1704 – 1782), were:
  • Susan (1735 – 1766)
  • Anne (1736 – 1738)
  • Philip (1738 – 1803), my fifth great grandfather
  • Anne (1739 – ?)
  • Jane (1742 – 1829)
  • a child of whom I do not know the name, born 1733 and presumably died young
Philip, the brother of the first baronet, had died in 1803, so who was the Philip who was imprisoned? My family tree currently has ten Philip Champion de Crespignys though many of these are quite obviously not the man referred to in this book. My fourth great grand uncle Philip (1765 – 1851), son of Philip the brother of the first baronet seems to be the right man.

Philip married Emilia Wade on 21 October 1809. She died in 1832.

Family history notes compiled by Stephen de Crespigny state:

Philip was in France when Napoleon’s detenu order was signed in May 1803 and was held at the fortress town of Verdun for some years , then moved to St Germain from where he escaped in May 1811. But during the time he married Emelia Wade in Paris in November 1809, a lady with a considerable fortune. (photocopy of handwritten notes on the family history compiled by Stephen de Crespigny and in possession of my father)

The Treaty of Amiens had been signed on 25 March 1802 and meant peace between the French Republic and the United Kingdom.  The peace lasted only until 18 May 1803 when Britain declared war on France.  Issues leading to the recommencement of hostilities seemed to include the British not withdrawing from Malta in accordance with the treaty terms. The Wikipedia article on the recommencement of war between France and Britain states:

On 17 May 1803, before the official declaration of war and without any warning, the Royal Navy captured all the French and Dutch merchant ships stationed in Britain or sailing around, seizing more than 2 million pounds of commodities. In response to this provocation, on 22 May (2 Prairial, year XI), the First Consul [Napoleon Bonaparte] ordered the arrest of all British males between the ages of 18 and 60 in France and Italy, trapping many travelling civilians. This act was denounced as illegal by all the major powers. Bonaparte claimed in the French press that the British prisoners he had taken amounted to 10,000, but French documents compiled in Paris a few months later show that the numbers were 1,181. It was not until the abdication of Bonaparte in 1814 that the last of these imprisoned British civilians were allowed to return home.(Treaty of Amiens. (2013, August 28). In Wikipedia, The Free Encyclopedia. Retrieved 07:15, September 1, 2013, from http://en.wikipedia.org/w/index.php?title=Treaty_of_Amiens&oldid=570571270)

In 1851 Philip was recorded on the census as living at Harefield House in Middlesex with his half brother, my fourth great grandfather, Charles Fox. Charles Fox Champion de Crespigny was twenty years younger than his brother. Philip was described as a lunatic, by which I assume he was suffering from old age dementia.

Source Citation: Class: HO107; Piece: 1697; Folio: 376; Page: 35; GSU roll: 193605Source Information: Ancestry.com. 1851 England Census [database on-line]. Provo, UT, USA: Ancestry.com Operations Inc, 2005.
Stephen’s notes tell us a little more of Philip:

It was said of him that he was a gay fellow with a happy talent for drawing. He lived for many years in Harefield House, near Uxbridge, where he kept a dozen or more servants. Towards the end of his life he became a little senile, and his half brother Charles Fox looked after his estate for [him (remainder of note missing)] (photocopy of handwritten notes referred to above)

Philip died on 22 May 1851, only a few months after the census was taken. He died without issue.
Harefield House was used as a military hospital by the Australian Army in World War I.
ID number P02402.005 Photographer John H Avery & Co Description Harefield, England. c. 1915-06. Exterior of `Harefield House’ the former stately home taken over by the No. 1 Australian Auxiliary Hospital (1AAH) in 1915-03. On the right is the corner of one of the first wards and hidden from view behind the shrubbery is the bay window of the first operating theatre (formerly the sitting room). (Original housed in AWM Archive Store) (Donor R. Brown).
Retrieved from http://www.awm.gov.au/collection/P02402.005  1 September 2013