My dog Murphy sees several different DVMs on a regular basis. Each has their strengths and each is playing a vital role in his health care.
That said, please don’t misunderstand what I write below.
We are the ones that advocate for our pets. If we don’t, they won’t get the care they need. I recommend, if your current vet is not able to diagnose the issue after all this time, he or she is not the right vet to see about it.
Find another vet or, find a well respect teaching hospital like the Royal Veterinary College. I don’t know where you live in the UK, so you’ll need to search for one closer to you. Teaching hospitals see a great variety of things that our vets never get exposed to.
I am fortunate to be an hour away from one of the best vet schools in America, CSU. We are frequent fliers up there.