Share valuation upon death of shareholder

During the probate phase I would think

Professional assistance needed

You need a specialist consultation with an accountant experienced in carrying out share valuations, and the assistance of an experienced probate professional - there are Inheritance tax implications arising from the valuation etc

It is even possible that the status of the shares for probate negates the need for a value for tax purposes, but you should still get one to demonstrate that the executors are fulfilling their duties to the estate and the beneficiaries.