I was pretty dissapointed with my 20 week scan as well..which seems really stupid as our baby has a problem with her kidney and they were looking hard at that and other organs, I guess thats what it is for...BUT... as a mummy you want and need that picture..
I would say go for another in the 3rd trim, maybe even a 3D, but that really depends on bubs position as to whether they can get a good picture as well.
Sorry i really feel your dissapointment.