An indemnity policy is an insurance policy which is put into place for a one off premium (usually based on the indemnity limit depending on the risk) and provides an indemnity fund in certain circumstances for the insured to use to help with problems which may arise.
They often have conditions attached and certain actions can void the policy so best to check with your solicitor before making any enquiries. If for a valid reason then it is usual for the seller to cover the cost.
A conveyance is a sale document which, if unregistered could be part of the proof of chain of ownership and whether the property is registered or unregistered at the land registry could provide info about the land such as restrictions (eg covenants) so it is covering the risk of the unknown in a sense.
For example if the buyer wanted to do something with the property such as use it for a business which the missing document does not allow then the person with the benefit of the covenant could try and enforce it.
It is common in a property purchase to check the planning history as if works have been carried out which require planning permission but for which there is none then the planning dept could in certain circs ask for the unauthorised works to be taken down.
Questions to ask yor solicitor could be was this looked into when you bought, have the deeds been checked in case they are with them, is there any evidence of consent being obtained on the local search obtained by the buyer?