self-build register query and statutory returns - Public forum - Planning Advisory Service (PAS)

Just wondering if anyone knows whether we, as a local authority, must write to everyone on the self-build register annually as to whether they want to remain on the self-build register. Also if we do and they no longer wish to be on the register, do we delete the whole of their data and thus their data is no longer counted for our statistical returns? I am concerned about GDPR and also ensuring we provide accurate statistical returns. Many thanks.