We took it into the dealer in the end, as they said we don't need converters or any adjustment, and reckon they have never had a problem driving a DS3 in France with Xenons, which have the same light pattern. They also said you can't put stickers on because the lights get so hot that it melts the adhesive on and the lights cost £3K to replace, and Citroen would have a product if they could (just to make money!). Apparently, no one else has ever asked, or had any problems not having converters.
However, I think we will get the stickers anyway so we are not blinding people, Halfords have them with instructions on how to fit to Xenon headights. There is the little "+" on the light so must be the same positioning as on the Halogen.
Thanks for your help