I've used the ombudsman before, I can't give links on here, have to make more posts or something, but go back to CAB, I have already established that your complaint is NOT a health issue. Your issue is to do with a bad service by a company who clearly gave you bad advice and not only have they given you bad advice, but they are currently trying to get more out of you. I'm not sure who you got your Advocate from? Like everything else, unfortunately, there are rogue people out there who claim to be representing you and will offer you an ombudsman service. My advice here is if anything wants your bank details, you are on the wrong page.
When I filled in the ombudsmen form, I didn't write to the company first, I used the online service to write to them. Focus on the false hope and the fact that they are still trying to get you to pay more money out. Don't contact the company in the meanwhile, the correct form from the ombudsman will make it clear what you should be focusing on with your complaint, and all complaints should be done via that form. What you don't want is a he said I said scenario. By the way, when you get there, and it asks what you want the company to do for you, go after a full refund from them, if necessary you can always compromise at a later date if you felt they actually did anything for you.
In the meanwhile, ring your local trading standard services in your area. Make it clear that your complaint isn't to do with the NHS. Focus on getting money back from the Advocate service, and that you suspect that they are using your stress to try and get more money out of you. Sorry, but they sound like rogues, and I bet most people on this forum will agree with that. Ask your local trading standards to email you the link to complain to a free ombudsman service, so you get the correct department. Certainly at this stage, make it very clear you are not trying to fight the NHS, you want refunding for a bad service who claimed they were working for you, and kept taking money from you, which you handed over because you felt there was no other way for them to start work for you, and that they kept encouraging you to give them money, and due to your stress, you fell for it and that this Advocate is now suggesting a further £7000 to be paid to them. This is a clear structure that is used by 419 scammers, and even if they are an official company, they are still using the same techniques.
Most CAB services have a volunteer solicitor come in, these are often people who aren't fully qualified as yet, but are fully versed up and keen to add to their CV about won cases. Cut out the middle man, ask your CAB if they have a volunteer solicitor come in sometimes and ask if you could make an appointment to see him/her. If they ask what it's about, tell them you feel that you have been scammed by a service and that you have already paid £15000 out, and they are suggesting a further £7000 and are willing to take your case on again if you agree to pay for it. I hope they at least had the decency to suggest that you shouldn't carry on when they asked for that money.
Even in this area, £15000 would give you 15 weeks of paid for private care.
If all that fails, go to a local solicitor, (not an online one) and ask if they take on any no win no fees cases, and would they be willing to take on a case of bad services given. Ensure that they collect any fees from the company and not from you. If they turn you down, then at least you know there's a strong chance you won't win, they won't take on a case unless they have a very strong chance of winning.