I've just found a website which promises to do it for you. The only problem is that they charge US$425 !!
I'll assume that you don't want that URL and suggest, instead, that you contact the French embassy for advice:
French Embassy in the United Kingdom,
58 Knightsbridge, London SW1X 7JT
Tel. 0207 073 1000
(France must be the only embassy that has a web page which includes information on everything from wines to comics but omits such mundane topics as banking! :
http://www.ambafrance-uk.org/rubrique.php3?id_rubrique=9 2 ).
Chris