First some context for thinking about what to do:
Almost all cavaliers have the malformation which alone can cause symptoms, and somewhere between 30-70% of research samples from around the world consistently show SM -- so it is perhaps better not to worry about whether a cavalier has SM, but whether it is symptomatic SM that is causing problems and pain for the dog.
Now:
If this is the case with Minnie, a 5 year old that only scratches on walks is showing mild symptoms and would not overly worry me, personally. I'd also want to see more than that, before I'd be considering the likelihood of SM or the need for an MRI.
There are a hundred things that could cause scratching. If it is really bothering her then please see your vet to start to eliminate all the more likely reasons she is scratching, including mites, fleas, allergies, and PSOM (very few cavaliers have symptomatic SM).
If she isn;t bothered than I really would not put her through a further series of far more invasive tests to see if she has SM -- an MRI will cost over €1000 in Ireland, requires a general, and will probably only tell you what most of us would find anyway with an MRI -- the malformation, probably some herniation of the brain as this is very common, and perhaps the formation of some syrinxes. If you travel to Chester they will not offer much help on treatment -- the mini MRI they do for lower cost is really targetted at breeders, not diagnosis and treatment of affected cavaliers -- most cavaliers will need a far more thorough MRI than the MRI for treatment and Chestergates will tell you this. Thus it makes more sense to stay in Ireland.
There is no reason to MRI a mildly affected cavalier IMHO but you can have her go through a neurological exam at UCD without an MRI, if your vet is unable to find any other cause for her scratching.
Then with the neurologist's agreement, you might put her on something like frusemide to lower CSF pressure and see if that alleviates the scratching.
There's lots of info on my SM website. My vets -- Anicare -- are familiar with SM, especially Tanis, Susan or Fintan, and so is Adrienne at Beaufield vets in Celbridge, so if you are really concerned go see one of those vets and have them go through the process of eliminating other causes.
I do feel quite strongly though that given the consistent research results over the past several years, people who own cavaliers need to realise that it would be unlikely for them to have a cavalier without CM (Chiari-like malformation) and there's probably at least a 50-50 chance that any cavalier has SM to some degree. It isn't a case of other people's cavaliers having this problem and hoping yours doesn't have it, but a case where 90% plus cavaliers having at least some of the problem (CM and often the associated issues like herniation of the brain) and half or more having SM as well over a lifetime. That is why this particular condition is so alarming and so threatening to the breed. No researcher has yet had a sample that didn't fit this level of affectedness on MRI.
This is the advice I give on what to do if you think your cavalier has SM:
http://sm.cavaliertalk.com/diagnosing/diagnosing/isthissm.html