Greta died 4 months after CHF. This disease really sucks. It is on top of the list of the meanest diseases a Dobie can have.
I am very sorry you have to go through this and I wish you strength for the upcoming process, as well as for the possible decision you have to make in the end.
I second, find a specialised cardiology vet. You do have them in England and they are affordable. Google, you will find them. If you do not find them, you can call the next big veterinary hospital at a bigger city or call the veterinary department at the next university. Or, maybe better, call an emergency vet. They usually have long lists of specialised doctors they refer to.
Why is that? Because the mix of medication your dog needs now is, as you already see, very specific. Only a cardiologist is state of the art and can give you the exact mix your dog needs. This is not the same for every dog.
All the best, keep on posting to this forum if you like, there is a lot of support here, especially regarding DCM.