The most important issue here is Heidi not your vet. Karlin has given out a link to a list several times of neurologists which specialise in sm. I would choose one, go to your vet and simply tell him (nicely) you want to be referred its not up for discussion. It is important Heidi is diagnosed and treated as quickly as possible as this disease is nasty and painful. My Molly has SM, I can tell by her expression in her eyes how she is feeling, I would follow your gut instinct which is probably right, if you don't you will regret it.
Please let us know how you get on and I wish you all the best.
Tania and The Three Cavaliers!
Dotty!- A Sweet Little Tri
Molly - Pretty Tri Dougall - Gorgeous Blenheim