If I understand your situation correctly the overage has arisen during the course of the administration when an assets held at the date of death was sold. The overage would have been part of the proceeds on the sale of the asset represents unknown additional consideration at that time. It is likely that the proceeds for the overage should be subject to CGT but there is potential for this to be subject to the transactions in land legalisation and fall to be taxed as income in certain circumstances. Julie Butler wrote an article here https://www.butler-co.co.uk/articles/tax-planning-and-the-overagepdf. You may also want to consider the sections in the CGT manual at CG72850P.
As to who the tax is payable by if the estate has been closed then presumably the overage has passed out of the estate and into the hand of the beneficiaries.