Hi Rosemary - I am sorry to read of your losses. Forms FR1/DL deal with the first registration aspect and form AS1 can be used by the Executors for your late Mother, as the surviving joint owner, to transfer the legal ownership to the beneficiaries as appropriate.
You'll need to also include a certified copy of your late Father's death certificate as well as the forms and more as explained in our online assisted guidance -
HMLR Guide Page · HM Land Registry