have you looked on the Landregistry website? It should tell you there. It may say which on the original mortgage arrangement or on the papers sent when the mortgage was paid off. Make sure you go to the ".gov site" as there are loads out there who will go to the site for you and charge extra fees, It cost £4 to get a copy of the registry entry if there is one.
You do know, don't you, that even if probate is not required, there is still an inheritance process to go through?