Thanks for your question.
If the deeds have been lost and the extra land is not registered then you will have to apply to the Land Registry to have this registered in your name by producing statutory declarations from you and others who can speak to the fact that you purchased the land.
You will receive what is term "possessory title" of the land, which is slightly different to the absolute title that one receives when you are able to produce the deeds to effect first registration. In this case the proprietor of the possessory title can apply to have the title upgraded to Title Absolute after 12 years if no person has attempted to register the land by using the deeds. Once you have title absolute the title is perfected and no-one can challenge your ownership.
You (or your successor in title - ie. The buyer) would receive notice of any person seeking to register title absolute if you have possessory title.
Ultimately, it's for the buyer to decide and they would ask that you pay the costs of an indemnity policy for lost title deeds (cost around £150-250 depending on the value of the landl), but if the land is only additional land to the actual property I would mildly surprised if they pulled out. Each person is different though.
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