Things may have changed since 6 years ago when I had problems with Scottish Power overcharging me.
Check with Trading Standards. Trading Standards were always very helpful when I rang them in the past. When you ring them tell them you are suspicious of the company that you dealt with, and their idea to help you was to re-appeal and pay them an extra £7000 to do that. If the Advocate firm insists that you have to ring them in order to go through to a complaints department, let Trading Standards know that before ringing the company, and tell Trading Standards you are afraid of being sucked in again to part with more money.
If you can write to them, keep copies of any expenses incurred, stamps, paper, envelope, est. ink price. Keep copies of everything you write and any reply they make. Don't pay out any money to anyone or give them your bank details, even if they tell you they can win for you.
If you go on moneysavingexpert they have a resolver complaints tool and template letters. It's tricky to scroll through because you need to ensure your complaint is about service rather than goods.
I'm not sure what the situation was with Social Services and cheques was, the only thing I can think of there, is that if he was living on his own they may have sold him the careline or lifeline service, or he may have had to pay for care from them to visit so many times a day after a hospital visit or because they felt he had to have visits to help him with some day to day tasks such as bathing, and getting up or going to bed. POA documents only come into effect when you have provided them with the original documents or a certified copy and registered it with them that way. I am mystified why they wouldn't just tell you that, if the case though. All cheques are crossed nowadays, so if a cheque was written, and you had POA and the bank knew that, I would have thought that they would be able to tell you which bank account the money was paid into, especially if it's paid into a company and not with a private individual.
I do know of a bank manager that was arrested for filtering out money from elderly people and putting the money in her own account, but this was done in large amounts, she got caught and was arrested for doing it, but that's another story.