Its a common question but one that you might not have a choice over, many mortgage lenders will (if a potential liability is bought up by a search) insist that an indemnity insurance policy is purchased and put in place, others may allow you to decide whether you wish to purchase an indemnity insurance.
You should always discuss the options with your solicitor.
A few factors you might want to consider when deciding whether you need an chancel repair indemnity policy:
How dense is the housing surrounding the property you are purchasing? If you are purchasing a property within a large built up area it could be that the potential liability (if discovered) would be very minimal, and likely that any potential claim could easily mean rather than fighting the claim alone you could be amongst hundreds even thousands of home and landowners fighting the claim.
Equally if the property of land is in a smaller settlement, the cost of the insuracnce policy is probably well worth the investment for the peace of mind that it offers.
