Rosella PLEASE take Tom to Mr Bate. Some of the things your vet has told you are either wrong or very dumbed down in his way of explaining.
He said stroke can be caused by clot elsewhere in body not brain which may not affect a cat's behaviour.
I did mention vestibular disease but vet said this is loosely linked to strokes? Sorry can't remember his exact words.
Both of these are wrong. If a clot occurs elsewhere in the body, blocking a blood vessel, it is called thrombosis, and has no effect on brain activity unless the clot travels to the brain. A blood clot or haemorrhage in the BRAIN is a stroke. Only if this happens in the brain does it have neurological symptoms such as Tom's.
Vestibular disease has nothing to do with strokes. There are various causes of vest disease such as ear infection, brain tumour, (in this case, there is no improvement), or no cause at all: idiopathic, which is what Mr Bate thinks Polly has had. He gave her antibiotics to cover for the possibility of infection. She's improving dramatically. She saw him earlier and doesn't need to go back as long as the improvement continues.
As you're so close to my vet's, I would highly recommend that you go to him for a second opinion. He's an amazing diagnostician and doesn't do any unnecessary tests. He'll look at Tom, give him a thorough examination, including watching him walk, and tell you what he thinks is wrong. The consultation cost varies. Follow ups are cheaper (£16 ish plus meds) than initial consults.
Samantha, Polly and me are all sending good vibes to Tom. I hope he gets better soon, whatever he's got.