there are lots of different types of scan, i had an oscar scan done privately, this is the combined scan and bloods for downs and other defects, i went with bupa which cost £196 and worth every penny, they were fantastic, so relaxed and helpfull and really put my mind at rest, i was also given the photos they took from the scan.
I am also going to have the 4d scan at 28 weeks again with bupa.