this is british form and does not correspond to french nobiliary rules which in any case are defunct given that french noble titles were rendered obsolete more than a century ago i think that technically she is merely raine spencer having retrieved her previous surname upon her divorce from chambrun and during the french marriage she was not countess of chambrun she was countess jean francois de chambrun and as per french usage would be referred to as mme de chambrun with the title used only by servants and so called inferiors