Thanks for the replies. Really helpful - feel in much less of a muddle now!
I think I might do what Kate did and get an initial assessment privately (so i can check that I'm not about to waste eveybody's time) and then ask to be referred back on the NHS.
I don't think my GP meant that they pull someone off the street to read MRIs but she did say that it needed to be looked at by someone who knew lots about hip impingement to be most helpful.
Is hip impingement quite a newly recognised thing?