A search fee of £6 will ensure that you receive a copy of the grant of probate (which includes a copy of the will) if probate has already been granted or as soon as it is granted during the next 4 years. (If you want the search to remain in place for a longer period there's a further charge of £4 per 4 year period).
You simply need to fill in this form and post it (with a cheque or postal order) to the address shown:
http://hmctsformfinde...r/forms/pa1s_0405.pdf
Assuming that the assets of the deceased person were held in a bank or, for example, as shares, the executor won't be able to access them without a grant of probate. (If the deceased person kept all their money under the mattress the executor still can't lawfully distribute the estate without probate but it's obviously easier for them to do so).
However if the assets of the deceased person were less than £5000, probate is not usually required (although banks, etc, may still insist upon it anyway before releasing the assets).
Chris