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As the Deeds are in your late Father's sole name, you will need to apply to the Probate Registry for Letters of Administration de bonis non. You will be applying as Executor of your Mother's Will and this Grant applies to the administration of estates that have not been completed by the time the original Executor has died.
Once you have received the Grant, you will then be able to sell the property.
If you appoint a Solicitor to deal with the Sale, their Wills department will be able to assist you in dealing with the application, unless you are wishing to do it personally. The Sale can still proceed, but it will just mean that you won't be able to exchange Contracts until the grant has been obtained (it is likely to take 3-4 weeks, and during which your Buyer's Solicitor will be putting in hand searches/raising enquiries with your Solicitor/ Buyer arranging any Mortgage, so it should all fit into place once the Grant has been received).
Nearly all Solicitors have a Wills/Probate department, so provided you instruct a local firm, as opposed to any "Conveyancing firm" the Agents may push you towards, it will cut down on any undue delay.
I hope this assists and answers your question.