Muggerrocks

Hi, my Niece inherited a property overseas and is planning to sell it in the coming months. We understand that he would need to pay for UK CGT by filling his sell assessment return. Can we understand what sort of record is required by HMRC for the record and do we need to submit it? Would the sale contract with transaction price suffice? Do we calculate the CGT on the transaction price or we need additional valuation done? Do we need to translate the contract if it is not in English? Thank you very much!