Andy - if the deeds are in your late Father's sole name then the property needs to be transferred by his executor and as such that triggers a compulsory first registration. So you will need ID1/conveyancer or ID3/others as listed in
PG 67 as appropriate for both the executor and your Mother
Not relevant now as it's a compulsory registration but it would only be a voluntary application if your late Father was still alive and being registered as the owner. And identity verification would then only be required if the deeds had been lost