Clicking on the link below will open up Land Registry Practice Guide No 2 which describes what process is required to go through to register title when title deeds (of a previously unregistered piece of land) have been lost.
http://www.landregistry.gov.uk/assets/library/ documents/lrpg002.pdf This is the seller's problem - not yours - and they should go through this process and pay the LR application costs.
The problem is that reconstituting a possessionary title in this way may omit any restrictive covenants (easements) that may have been associated with the original title. Its often not that the person claiming ownership of the land is trying to be dishonest - they may not even know about an obscure easement set up when a piece of land was conveyed many years ago. Because of this the LR will place a caution on the LR title, making this point.
I suspect that this indemnity insurance is required to protect against another land owner emerging out of the woodwork and claiming the benefit of the original easement some time later. The insurance typically covers the deficiency in the title - not a specific length of time. But its worth checking that it doesn't expire when you sell up.
Why isn't your solicitor telling you this stuff?