I opted for a 3D/4D scan at 30 weeks which is the latest recommended at this particular clinic.
Would highly recommend if you have the money spare.
The appointment lasted 45 minutes and it meant we could ask lots of questions which the sonographer was then able to show us on screen (as she was explaining). When the baby moved into a difficult position, we were sent for a walk so baby would turn again and we could carry on.
We got a DVD of the whole appointment which will be a fantastic momento plus 30 prints and a disk with an additional 70 pictures.
20 weeks is a long time to wait between last NHS scan and meeting baby (unless you have any irregularities that need further investigation) and the reassurance we got was well worth the money!