***Remember, I am not a neuro - I don't even have a medical degree!***
The diagnostic criteria for MS are dissemination in time (which basically means more than one attack) and dissemination in space (which basically means more than one part of the central nervous system being affected). TM is considered a clinically isolated syndrome, which means that it is a single attack affecting a single part of the nervous system (i.e. the spine). The dissemination in space criteria can be satisfied by having a single lesion in a typical MS area in the brain as well as a single lesion in the spine. From what you know about your MRI, you have more than one lesion in your brain. Unfortunately, there's no way of knowing precisely where they are without looking at the images so it's impossible to say whether or not they "count" towards the dissemination in space criterion for MS. If they do, then a diagnosis of "probable MS" would be more appropriate. [We also don't know just how long that spinal lesion is. The description "at C1" just doesn't fit.]
Your vertigo episode is very typical of MS. Of course, relapses vary massively, but the gradual build up and gradual recovery is very familiar. The dissemination in time criterion can be demonstrated by a new lesion in the absence of new symptoms, but that is no reason to insist that a relapse is only demonstrated by a new lesion. In fact, that is b*******! NHS MRI is simply not good enough to pick up many lesions and some relapses affect nerves that do not show up well on even good MRI. I really don't get your neuro's dismissal of it. I also don't get the whole discharging you thing either - I think the neuro should have at least referred you to an MS nurse.
You can request a copy of your scans and the radiologists' reports from the hospital that they were done at. There is a charge when we get them ourselves (£20 or so usually), but your GP may be able to get them for you for free. Worth asking at least!
Would I have a second opinion if I were you? If the MRI reports suggest probable MS rather than TM and this neuro has discharged you, then I think yes - at least you might get put on someone's list properly. Perhaps your GP can advise though? Maybe there is some local thing behind the discharge and you will be able to see the neuro quickly if something else happens despite what it says on paper? I don't get the lack of an MS nurse though. That's what I would be pushing for, more than anything. Neuros get seen occasionally. MS nurses are the ones we & our GPs work with to get meds, etc worked out. They are the ones that we report relapses to and they are the everyday conduits to our neuros. If you have a really on the ball, supportive GP, then it isn't such a big issue though. Sorry, this is all very woolly!
There is a chance that it could be cleared up by your GP writing to the neuro to ask for his opinion on your status versus the dissemination in time/space criteria (taking into account your MRI results and your episode of vertigo, that he himself observed) and also requesting that you are referred to an MS nurse to get support for ongoing symptoms. [Your GP may be able to refer you to the nurse btw - worth asking.] If your GP doesn't want to get involved, you could try writing yourself.
The most important thing you should read and understand is the Polman et al (2011) paper on the 2010 revisions to the McDonald criteria (the "gold standard" diagnostic criteria for MS). You can download the pdf by clicking on "Get PDF" from here: http://onlinelibrary.wiley.com/doi/10.1002/ana.22366/abstract
If I were you, I would take a copy of this along to the GP. He may not know about it and it may help to get him on your side about getting your diagnosis and ongoing support sorted out.
Good luck with the GP.