I think most people need more than one, don’t they? I myself had four or five (lost exact count), and that was even when they did all show abnormalities. At the time of my earliest ones, nobody had even suspected MS - they just thought I had a slipped disc. So the scans weren’t really optimized to look for MS-like damage. Nevertheless, they did spot something odd - so they had to be done again, with different settings, to try to get to the bottom of what it was.
I’m afraid it is impossible to prove categorically that you don’t have MS. The best answer you’ll get is the one you’ve already had, which is that they haven’t been able to find any evidence of it. Although, in the early stages, it’s possible for MS-related damage to be too subtle to show up on MRI, in practice, the longer you’ve had symptoms, the less likely this would be - in other words, it doesn’t stay invisible indefinitely! So the longer it goes on, and the more scans you’ve had, and they’re ALL coming out clear, the less likely that you have MS.
That certainly doesn’t mean you’re imagining things, or making them up, but it does suggest the answer may lie elsewhere.
Be very cautious about paying £100s for a private scan. Private does not automatically mean better or more powerful. Unless you know for sure the scanner they’d be using is more powerful than the local NHS machine, best to assume it isn’t. As far as I know, the scans I had on BUPA were only 1.5 Tesla (the measure of strength for MRI scanners), which is exactly the same they use in most NHS hospitals. So they wouldn’t have “seen” anything the NHS couldn’t.