Do you have copies of the contract/transfer and registered title which you can upload?
Difficult to advise without sight of the paperwork. But if the transfer is at the Land Registry it is too late. The transfer has completed.
You can try to negotiate with the buyer and ask for the application be withdrawn, refund the money paid. If the buyer does not agree there is little you can do.
Depends on what terms were in the agreement. As yet all I know I the buyer paid £10k for a small part of the land.
If the document which is at the Land Registry includes terms to buy the remainder at a later date then the buyer can possibly do so.
Can you not get a copy of the document? Try to get a copy from the buyer or land registry.
You should have a copy to have a record of shat you agreed with the buyer.