It could be a Defective title indemnity, meaning that there could be a defect due to the lack of planning permission - the back wall could have been built on Common land for example and there could be rights of way that have been blocked as a result of the building of the wall.
The link here may help
http://www.royalsunconnect.co.uk/preloginprofi n/LegalIndemnities.asp
As regards premiums, should be a minimum premium, although minimum premiums on these types of covers could be �250 or so ?? worth giving them a call.