Having a scan because something else is suspected is a bit different from having one just because you have MS.
Presumably, you have discussed your concerns with your existing neuro? Did he or she feel there were other things going on, that aren’t adequately explained by your MS diagnosis? Or that it’s all the MS?
And did you go back to your GP to chase about the missing appointment?
If you really do think you have something else going on, that has nothing to do with MS, then your best bet will be to take it up (again) with your GP, as your MS nurse, by definition, isn’t going to arrange investigations for unrelated problems.
However, her view is still important. In her experience, does she feel your symptoms are consistent with MS, or does she agree with you there’s something else going on?
If your current neuro AND your nurse both agree there’s nothing happening that can’t be explained by your existing diagnosis, it’s probably fruitless pressing for further investigations. As others have said, it will just come out: “Yes, you do have further lesions, and that’s to be expected.”
But if your GP thought there was something else in the mix, time to revisit that avenue, I think.