Hi - In most cases I would think this was a strange request, but as you state this is a flat then the insurer could be simply ascertaining the burden of responsibility to repair the said damage, or enquiring if it's a shared liability by looking at the deeds..
You can download a copy of your title deeds for £3 direct from the HM Land registry website, saving you £57
http://www.landregistry.gov.uk/wps/portal/Property_Search