South African Drivers License (Cannes)

Hi All South Africans living in France...
I have a South African drivers license and lived 12 years in Cape Town, and while I know that it can be swapped for a French one within the first year of living in France... my local prefecture is asking me for a letter from the British Embassy in SA to state that I lived in South Africa for more than 6 months during the year that I got my license, and that I have now left the country (South Africa).
I have tried giving the prefecture all sorts of other documentation to show that I lived in South Africa at that time, but they are insistent on this particular letter... which the British Embassy will not write. (I have a British Passport)
Does anyone have any advice of who I can turn to - or where I might be able to get a letter of this sort?
Thanks in advance!
Alice