Mortgage discharge - Death of mortgage lenders

I and my three siblings (two brothers and a sister) took a registered charge over our mother's house to secure a loan that we made to her.  The house is now being sold and we will need to release the charge.  Sadly, my brothers, Tom and John, have both now died.  I understand that my sister and I, as the surviving mortgagees, cannot give a valid receipt for the mortgage but the PRs of our brothers' estates will need to be a party to the mortgage discharge.  I have (or can obtain) death certificates for both brothers but I believe that there is no Probate or Letters of Administration to John's estate.  
It may be that John's widow has been able to deal with his estate without obtaining a grant of representation (if, for e.g., all assets were jointly owned and passed by survivorship).  I am very reluctant to tell his widow that we need a grant for this one specific purpose - quite apart from the fact that this is likely to delay the sale for a number of months before Letters of Administration are issued by the Probate Court.
Is there a practical solution to this?  Thank you

Richard - the charge does not form part of your late brothers's estates so you would not need probate or letters of A for them. You don;t state where your understanding came from but the discharge can be completed by the surviving siblings, namely yourself and your sister. An official/certified copy of the two death certificates would be required to confirm their deaths and why just two of the named lenders are discharging the charge

Adam - that is very good news and thank you for replying so promptly!
Richard