Lifting charges on deceased's house from firms no longer in business

I am the executor of my cousin's husband's will. The main part of his estate is his half of their house (worth in total £100,000). In the 1980s two charges were added to the deeds of the house, as he owed two firms small amounts of money (under £500 each). I have sorted everything else out (a long process), and am now trying to sort this out. Both the two firms have ceased trading and no longer exist. I have found family members of the people who owned the two firms, but am now stumped! How can I get the charges lifted? One of the people I have found (the widow of the owner of one of the firms) has said that she will forgive the debt, but how can I prove this, and who do I prove it to? And who do I pay the debt to the other firm to, and how do I prove it has been paid and get the charge lifted?
I have dealt with everything myself to save my cousin having to (she is in her seventies, and totally bewildered by anything like this). There will be hardly any money left in the estate once other debts have been paid, and I want to avoid involving a solicitor, as there is no money in the estate to pay for one.
Please, if there is anyone who has a clue about this (Land Registry? Helpful property lawyer?), help us!