Roz
Have you considered a complaint to the Information Commissioner and / or the Insurance Ombudsman?
The Information Commissioner might be a long shot, as how can data on flood plains, accurate or not, be personal data? Well if it is linked to you, as in this case, I suspect it could be argued to be so. It's worth noting that 'third party referencing' has been banned (the practice of disallowing credit because another current or previous resident of your property had a blemished financial character). In that case the true facts about someone else are not permitted to be associated with you. In this case, falsities about flooding are being linked to you causing you financial disadvantage. One of the information principles is accuracy.
Good luck
d